Small antenna for receiving signals from constellation of satellites in close geosynchronous orbit

ABSTRACT

A C-Band or Ku-Band satellite communication system uses a relatively small receiving antenna while operating within current FCC designated bandwidth and using existing satellite configurations. Aperture synthesis techniques create nulls in orbit locations from which potential interference is expected. Bandwidth inefficient modulation techniques reduce transmission power flux density. Video compression reduces the power necessary to transmit video information. These three features make possible a receiving antenna with a receiving area equivalent to that of a three foot diameter dish, at C-Band frequencies. Comparable reductions are possible for Ku-, Ka-, S- and L-Band systems. Compressing the data reduces the required transmitted power by a factor of ten. Spreading the bandwidth reduces the power density below the FCC limitation. However, reducing the antenna diameter increases the beam width of the antenna, hence, the smaller antenna can no longer discriminate between adjacent C-Band satellites in their current orbital configuration. By designing the receiving antenna with nulls in orbital locations where potentially interfering satellites would be located, the small antenna avoids this interference. The same general technique is possible for a Ku-Band Antenna system. The FCC power limits are higher at Ku-Band than C-Band, however, losses due to rain absorption and thermal noise are higher at Ku-Band frequencies. Nevertheless, equivalent size savings on Ku-Band antennas are possible with the combination of the above techniques, when tailored for the Ku-Band environment.

This is a continuation of application Ser. No. 08/259,980, filed Jun.17, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates generally to satellite communicationsystems, and more particularly to a satellite communication system forcommunicating signals from a satellite within a constellation ofsatellites to multiple terrestrial antennas, which satellites transmitdifferent signals simultaneously at designated frequencies, such as C-,Ku-, S-, L- and Ka-Band frequencies, but which frequencies are oftennearly identical. The present invention also relates generally toterrestrial antennas for receiving signals from satellites, and moreparticularly to a terrestrial antenna for receiving a signal beingtransmitted from a satellite within a constellation of satellites, whichtransmit television signals in designated frequency bands, such as C-,Ku-, S-, L- and Ka-Bands. Finally, the present invention relates to thecomponents for use in the above mentioned satellite communicationsystem, such as the receiver, the transmitters and the associatedsatellites.

Television has evolved from a local broadcasting concept to a system inwhich a viewer may receive television signals from a variety of sources.Today, television viewers receive programming from at least one ofseveral different methods, such as direct "over-the-air" broadcasts fromlocal television stations, transmission over land cables, i.e., cabletelevision (CATV), transmission over microwave systems, and direct tohome (DTH) broadcast via satellite.

Television viewers may receive DTH satellite broadcasts by purchasinghome satellite dish equipment, however, current satellite televisioncommunication systems operate with receiving antennas that arerelatively large, e.g., on the order of 10 feet in diameter or more forcurrent C-Band parabolic dishes. At lower satellite frequencies thereceiving dishes are even larger.

The consequences of large receiving antennas affect the very nature ofthe type of service provided via satellite. Large dishes require aconcrete pad for support, large amounts of space, installation bytrained technicians and complicated positioning mechanisms due to theweight of the antenna, all of which translate to high costs for theinitial installation. The high installation costs directly impact salesas many consumers cannot afford these high installation costs.

While cost is a large factor, it is by no means the only disadvantage ofcurrent DTH satellite service. Many consumers dislike the aesthetics ofa large satellite dish sitting in their yard. Consequently, manyconsumers who could otherwise afford to subscribe to current DTH servicedo not subscribe because they do not want to place a large parabolicdish antenna in their yard. Due to the poor aesthetics of theseantennas, restrictive covenants in housing developments often prohibithome owners from erecting them.

The combination of high costs and low aesthetics of these antennaslimits the appeal of DTH satellite broadcasts, which directly competeswith current CATV providers, hence the growth of CATV has beencomparatively explosive due to lack of effective competition. However,CATV will probably never be available to all consumers due to its highinstallation costs in rural areas. Furthermore, the high costs of CATVinstallation means that many third world countries will not get CATV formany years, if ever. Thus, there will probably always be a market forDTH satellite services.

Even if the costs and aesthetic problems were solved, large antennas arenot practical. While large dish antennas may be suitable for use in someapplications, they are much too large for general home consumer use, orat least for use in most homes. The problem is particularly acute inurban areas, where it would be impractical for everyone to employ such alarge antenna due to space limitations. As a result, CATV enjoys arelative monopoly on television services in urban areas.

In some parts of the world, other DTH services called Direct BroadcastServices (DBS) are available. The major advantage of these systems isthey transmit signals at Ku-Band frequencies, which are higher thanC-Band frequencies. Higher transmitting frequency permits a smallerreceiving antenna, which for Ku-Band systems is on the average about 3feet in diameter.

While higher frequency signals permit smaller receiving antennas, eventhese antennas can be too large for some applications where space is ata premium. Thus, there is a need for reducing the size of televisionantennas, particularly at lower satellite frequencies, such as C-Bandfrequencies or lower.

In addition, the demand for television services from satellites hascaused the Federal Communications Commission (FCC) to approve narrowerspacings in the synchronous orbits, about 22,000 miles above the earth'sequator. The use of ±2° spacing allows many satellites to supplytelevision service to the U.S. market. As more and more DTH servicesbecome available, demand will cause further reductions in satellitespacing, making the problem of interference from adjacent satellitesmore acute.

Previously, it was believed that C-Band satellites because of theirpower limitations and close spacing (about ±2° ) in a synchronoussatellite orbit were limited to receiving antennas at least 8 feet indiameter, and in most areas, commonly 10 feet to 15 feet in diameter.These large antennas are commonly used today, and more than 4 millionsuch antennas are installed throughout the United States. These antennasreceive television programs from up to 18 satellites. If satellitespacing reduces further, larger receiving antennas will be required todiscriminate between the desired satellite and its closest neighbors.

Prior to the present invention, the only way to reduce the size of thereceiving antenna for DTH systems to something below three feet indiameter was to use a higher radio frequency hand, such as Ku-band(about 17 GHz), which is allocated by the FCC for direct broadcastservices (DBS). In this radio band, the FCC permits higher powersatellite transmissions, which translates to a reduction in the requiredantenna size. The higher frequency also results in a smaller width ofthe antenna sensitivity beam, as a result of the relationship betweenthe width of the beam and the radio frequency. For example, a two tothree foot diameter antenna operating at Ku-band frequencies, using abeam width of approximately 1.30° to 1.50°, typically can achieve anantenna sensitivity pattern that is sufficient to isolate signals fromsatellites that are ±2° from the targeted satellite.

The move to higher frequency, however, comes at the cost of a need foreven higher transmission power due to rain absorption at these higherfrequencies. Rain has two effects on radio waves passing through it.Rain scatters the energy so that less of the energy reaches thereceiver, and rain radiates thermal energy that reaches the receiver,thus increasing the random noise that interferes with the receivedsignals. The amount of absorption and increased thermal noise fromscattering is more severe for radio signals at higher radio frequenciesand therefore with shorter wavelengths. The overall effect of rain lossdepends on the level of rain expected and the reliability required forthe service. For typical reliability levels of DTH service, at Ku-Bandfrequencies, one must increase the radiated power by a factor of ten toallocate for rain loss. About one third of the increase is due toincreased noise and two thirds is due to rain absorption. For lowerlevel frequency bands, such as C-Band, the corresponding allocation ofpower increase for the same level of reliability amounts to only about30%.

By increasing the satellite transmission frequency to Ku-Band, higherpower can be transmitted from the satellite, and a smaller antenna willachieve the required isolation for a ±2° satellite spacing. For example,a three foot antenna operating at Ku-Band has a beam width ofapproximately 1.80°. However, Ku-Band also requires a tenfold increase(1,000%) in transmitted power to overcome losses due to rain. At C-Bandan increase of only 30% is typically needed for rain loss. Thus, merelymoving to a higher frequency does not necessarily solve all the problemswith antenna size.

In addition, to implement a small receiving antenna using existingC-Band satellites would seem to violate basic limitations on power andbeam isolation. The restriction on total satellite power is set by theFCC at -152 dBW/m² per 4 kHz bandwidth power flux density reaching theground. The FCC limits vary with frequency. A higher power is permittedat Ku-Band frequencies. In fact, no limits exist for frequencies in theKa-Band. The FCC limits are designed to protect ground microwave relayequipment from interference by satellite transmissions. Obviously,foreign governments have their own limits on radiated power. The presentC-Band satellites operate with radiated power up to approximately 36 dBEIRP, which falls just below the FCC limit when reaching the ground. Thenormal way to achieve a ground station antenna area reduction is toincrease the satellite power by an equal amount. A reduction from a nineor ten foot satellite antenna to a three foot antenna would normallyrequire a tenfold increase in satellite power, which would significantlyexceed the FCC imposed limits by approximately a factor of ten.

Reducing the antenna size and increasing the transmission power, even ifpermitted by the FCC, would not completely solve the problem because asmall receiving antenna has a larger directional receiving range. Asmaller antenna of normal design will receive the signal from thesatellite of interest, but will also receive interfering signals fromother satellites in the constellation, at least as currently configuredin the C-Band system, for example. The received signal will thus be sodistorted as to impair proper decoding and reception.

Thus, the other barrier to antenna size reduction is a correspondingincrease in the beam width of the receiving antenna. Current eight footC-Band antennas have beam widths typically of 1.8°, which is sufficientto discriminate between satellites ±2° away in orbit. A normal threefoot antenna has a beam width of approximately 4.9°, which is notsufficient to discriminate against satellites at ±2° from the targetedsatellite.

The power and beam width limitations are the main barriers that haveprevented the industry from offering television services to smallantennas at C-Band, which has in turn limited the growth of the DTHindustry. To offer DTH service to small C-Band antennas, both power andbeam width problems must be solved simultaneously.

Thus, the present invention is directed to the problem of solving thepower and beam width limitations necessary to reduce the size of thereceiving antenna in a satellite communication system. The presentinvention is also directed to the problem of developing a satellitecommunication system that permits the use of a relatively smallreceiving antenna, yet operates within the current FCC power limitationsand with existing satellite configurations, which system will operate inat least C-, Ku-, S-, L- and Ka-Bands. The present invention is alsodirected to the problem of developing a terrestrial antenna for use inthe above communication system that is relatively small, yet permitsreception from existing satellite communication systems, withoutrequiring a change in the FCC satellite transmission power limitationsor a change in orbital locations of the satellites. Finally, the presentinvention is directed to developing the components for use in the abovementioned communication systems.

SUMMARY OF THE INVENTION

The present invention solves these problems by using a combination of:(1) aperture synthesis to create nulls in the antenna pattern thatcorrespond to orbit locations from which potential interference isexpected; (2) spectral shaping techniques to reduce transmission powerflux density and interference; and (3) video compression techniques toreduce the power necessary to transmit video information. As used hereinthe term nulls refers also to minima in the antenna pattern, i.e.,places where the antenna pattern achieves minimum values. Thecombination of these three techniques permits the use of an antenna witha receiving area equal to that of a three foot antenna at C-Bandfrequencies, and antennas with receiving areas that are significantlysmaller than what is currently available for Ku-, S-, L- and Ka-Bandfrequencies, as well as other frequency bands.

In fact, the present invention permits a reduction in receiving antennaarea, from what is currently available, for any signal being transmittedfrom a satellite within a constellation of satellites, particularlywhere a reduction in receiving antenna area would cause the receivingantenna to be unable to discriminate between satellites in theconstellation. Furthermore, the present invention allows a receivingantenna to discriminate between a desired signal and potential noisesources, where the desired signal and potential noise sources havepredetermined physical locations with respect to each other.

The exact implementation of the aperture synthesis technique variesslightly in the different frequency band systems, whereas the videocompression technique and spectral shaping technique remain generallythe same. Each technique will be separately described, and then also setforth in an embodiment for a particular application, such as C-, Ku-,Ka-, L- and S-Band.

Video Compression

The same video compression technique applies to all systems describedherein, since the video compression used allows a reduction intransmission power by a factor of ten, regardless of the transmissionfrequency by reducing the required data rate by the same factor. Whilethe video compression technique used in the present invention by itselfdoes not form part of the present invention, its use in combination withthe aperture synthesis and spectral shaping, as well as the componentsthat result from such a use, are novel. The present invention employs acommercially available video data compressor available from ScientificAtlanta. The compression technique required for the present inventionneed not be this precise product, but may be any technique that achievesat least the same reduction in data rate. Obviously, as compressiontechniques improve, further reductions in transmission power will bepossible, thus further enabling a corresponding reduction in the size ofthe antenna, or a reduction in the radiated power, or perhaps anincrease in the number of transmitted channels.

Spectral Shaping

Another component of the present invention that allows rapidimplementation of small antenna service is the deliberate choice ofspectrally inefficient modulation. The data rate of digitally compressedvideo is three to five megabits per second (3-5 MBPS). This data ratecould easily be transmitted in a radio frequency bandwidth of 5 MHz orless, using an efficient modulation choice, such as Quadrature PhaseShift Keying (QPSK) or Quadrature Amplitude Modulation (QAM). However,use of QPSK or QAM would for some C-Band satellites violate the powerflux density limitations set by the FCC, due to the high spectraldensity of these modulation schemes. In addition, the use of QPSK or QAMcould also possibly upset coordination between satellites now used bythe industry to avoid inter satellite interference.

The present invention uses a Shaped Frequency Shift Key (SFSK)modulation scheme to keep the energy of the transponder spread smoothlyover the bandwidth of the satellite transponder, e.g., spread over the30 MHz for current C-Band satellite transponders. By spreading thebandwidth from 5 MHz to 30 MHz using the SFSK modulation technique, thepower density lies below the FCC limits. The spectrum, if anything, issmoother than the spectra in the present satellites and thus will causeless interference to users of other transponders in nearby satellites.By using SFSK modulation, in combination with video compression, theradiated power of the communication system of the present inventionmeets the FCC power flux density limitations. Yet, the present inventionalso permits a quick and simple transition from current service usingexisting transponders to the system of the present invention without anyincrease in interference to other users and without requiring newsatellite launches.

By choosing the optimum demodulation technique, the receivers of thepresent invention are also less susceptible to interference fromadjacent satellites, whether the other satellites carry signals that aredifferent than the SFSK signal of the present invention, or carry thesame signal as the SFSK signal of the present invention. The SFSKsignals themselves have a "coding gain" (or protection) approximatelyequal to the transponder bandwidth divided by the data rate. Thisamounts to a protection factor of between three and ten depending on thenumber of television signals of the present invention in one satellitetransponder.

For example, where the data rate is between 3-5 MBPS and the availablebandwidth is 30 MHz, the gain becomes: ##EQU1## This coding gain is notspread spectrum gain but is more related to earlier frequencycoordination techniques. It is basically determined by Shannon's Law,which relates the data rate, r, to the bandwidth of the modulatedsignal, B, by the relationship: ##EQU2## where C/N is the ration ofcarrier power to noise power needed to receive the signal. By itself,the use of SFSK modulation is not sufficient to protect against adjacentsatellite interference, but is does form part of the overall protectionof the present invention by reducing the requirements of the depth ofthe antenna sensitivity nulls and pointing accuracy required of a smallantenna.

Three different SFSK modulation shapes are available, depending onwhether the transponder of the present invention uses one, two or threechannels, which depends on the radiated power available in thetransponder. A transponder radiating 30-31 dB EIRP can transmit onetelevision channel using one particular SFSK shape; a transponderradiating 31-33 dB EIRP can transmit two channels using a different SFSKshape; and a transponder radiating 35 dB or more EIRP can transmit threechannels using a third shape, which is different than the other two. Thechoice of specific SFSK shape ensures that the present invention isnon-interfering with present satellite users and non-interfering withitself.

While SFSK modulation is known, and does not by itself form part of thepresent invention, the combination of SFSK modulation, video compressionand aperture synthesis is novel.

The adjustment of data rate with EIRP, while keeping the bandwidthconstant and yet automatically accounting for the reduction intransmitted power by providing additional protection through the choiceof SFSK modulation is also novel. Decreasing the radiated power for thesame bandwidth, while simultaneously decreasing the data rate,effectively increases the "coding gain". Thus, the present inventionautomatically provides additional protection from interfering signals.The result is that maximum channel capacity is achieved with a givensize antenna but unequal satellite EIRP's. For example, an interferingsatellite with 35 dB of radiated power would be three times moreinterfering to a satellite with a 30 dB EIRP, but the coding gain of thereceiver for the 30 dB satellite would have three times the coding gainto compensate.

Aperture Synthesis

The present invention employs an aperture synthesis technique to permita small antenna to discriminate between a satellite within aconstellation of satellites, despite the fact that its beam width iswider than the spacing of the satellites in the constellation. Aperturesynthesis refers to shaping the antenna not in a circle but in anirregular shape that puts nulls in the antenna pattern that correspondto precisely the orbit locations from which interfering signals areexpected to originate.

The aperture synthesis technique of the present invention places gaps inthe antenna surface to cause signals from satellites other than thetargeted satellite to cancel themselves out, while enhancing thenon-interfering signals. The exact design of the antenna will differ foreach frequency band for which it is implemented, however, the basicconcept remains the same. By matching the gaps in the receiving antennato the point at which the signals from adjacent satellites will impingeupon the receiving antenna, such that the interfering signals willcancel themselves out, the antenna effectively places notches in itsbeam width where interfering satellites are located.

The use of aperture synthesis to create nulls in the receiving antennasuch that the antenna cancels out interfering signals from adjacentsatellites is novel and forms part of the present invention. Additionaldetails of this aperture synthesis technique will be described below.

C-Band Satellite Communication System and Antenna

The C-Band satellite communication system of the present invention onlyrequires an antenna having an area equivalent to that of a three foot orless diameter parabolic dish to adequately receive the signal fromexisting C-Band satellites in their current configuration, yet stayswithin the FCC power limits of radiated power from the satellite thatreaches the ground. To meet the power limitation of approximately 36 dBEIRP at C-Band frequencies (3.9-6.2 GHz) in a relatively small receivingantenna, the present invention employs a combination the above videocompression and spectral shaping techniques. By compressing the data,the required received power is reduced by a factor of ten. Thus, withinthe same power limitation on radiated power from the satellite of -152dBW/m² in a 4 kHz bandwidth, an antenna with one tenth the area can beused.

The C-Band satellite communication system includes a small receivingantenna to receive conventional C-Band satellite transmissions. Due tothe combined use of the above three features, however, the antenna ofthe present invention can have an area on the order of that of a threefoot diameter dish, which is much smaller than any C-Band satelliteantenna known to be in use today for receiving television.

By reducing the antenna diameter, the beam width is normally increased.Reducing the diameter from 8 feet to 3 feet increases the beam widthfrom 1.8° to 4.9°. The result is that the smaller antenna can normallyno longer discriminate between adjacent C-Band satellites in theircurrent orbital configuration.

The present invention solves the beam width problem by designing thereceiving antenna with nulls in its antenna pattern that correspond tothose orbital locations in which potentially interfering satellites arelocated. The nulls are specific to C-Band frequencies and are located inorbit directions ±2° to ±4° from beam center, where adjacent satellitesare located.

To create the desired nulls, the present invention employs the abovementioned aperture synthesis technique, i.e., shaping the antenna not ina circle but in an irregular shape that puts nulls in its antennapattern that correspond to precisely those orbit locations whereinterfering signals are expected to originate. Nulls in the specificlocations unique to the satellite spacing at C-Band address the specificproblem of the C-Band television industry in a way that allows abreakthrough in service offerings. Nevertheless, the present inventionis not limited to C-Band implementations, but serves to significantlyreduce the antenna size for any satellite frequency and satellitespacing.

The third component of the present invention that allows a rapidimplementation of DTH service to small antennas is the deliberate choiceof the spectrally inefficient modulation technique discussed above. Thepresent invention uses an SFSK modulation scheme to keep the energy ofthe transponder spread smoothly over the bandwidth (about 30 MHz) of thesatellite transponder. By using the SFSK modulation, the presentinvention meets the FCC power flux density limitations and allowsimplementation of the present invention in existing transponders withoutany increase in interference to other C-Band users. Thus, switching fromcurrent DTH service to the new service of the present invention can beeasily accomplished. By choosing the optimum demodulation technique, thereceivers of the present invention are also less susceptible tointerference from adjacent satellites, whether the other satellitescarry normal C-Band traffic or the signal of the present invention. Bychoosing the data rate and number of television channels to vary witheach satellite's power level, the interference is equalized betweensatellites and the number of channels is optimized for a given antennaarea on the ground.

Although the SFSK signals themselves have a coding gain, which isapproximately equal to the transponder bandwidth divided by the datarate, by itself, SFSK gain is not sufficient to protect against adjacentsatellites at ±2° spacing. It does, however, form part of the overallprotection of the present invention by reducing the requirements of thedepth of the antenna sensitivity nulls and pointing accuracy required ofthe small antenna.

The combination of reduced data rate due to the video compression andthe coding gain provided by the SFSK modulation reduces the depth of theantenna nulls required to achieve a significant reduction in antennasize. Nevertheless, sizable nulls are still required. At least 10 dBnulls in the direction of each interfering satellite are required toachieve the necessary isolation of the desired signal from theinterfering satellite signals. The aperture synthesis techniquedescribed herein accomplishes the required 10 dB nulls in the directionof each interfering satellite.

The unique combination of digital television compression to reduce theoverall power requirements, antenna beam synthesis to notch outsatellite interference from adjacent satellites in the constellation,for example, from ±2° and ±4° positions in synchronous orbit in thepresent C-Band configuration, and SFSK modulation to reduce intersysteminterference, allows significant improvements in satellite televisionDTH service, especially in C-Band. The present invention allowstelevision programs to be offered to small-aperture user antennaswithout any change in the existing C-Band satellites. It allows theequivalent of direct broadcast satellite service (DBS) to be offeredwithout any launch of new satellites. The current satellites can beswitched from current service to the system of the present invention onetransponder at a time, as C-Band DTH users develop without interruptingexisting service. This smooth transition without requiring any newsatellite launches provides a major economic advantage to the system ofthe present invention. In fact, the cost and delay inherent in anysatellite launch forcloses implementing many other DBS system designs.By permitting a quick transition from previous service to the service ofthe present invention, without the huge cost of a satellite launch, thesystem of the present invention can be rapidly implemented in themarketplace. Furthermore, the C-Band system of the present inventionmaintains a permanent advantage of reduced rain loss, giving a tenfoldreduction in satellite transponder power required and a continuous majorcost advantage to the C-Band system of the present invention incompetition with Ku-Band DBS existing service.

The combination of these three techniques allows a C-Band antenna designhaving an area equivalent to that of a three foot diameter dish, ascompared to existing C-Band antennas which vary between 8 and 10 feet indiameter. Due to the small size, the C-Band system of the presentinvention does not require an installation professional to install theantenna or a concrete pad to support it. Finally, the aesthetics of theantenna are improved by allowing the user to locate the antenna in aconvenient location wherever the antenna has an unobstructed line ofsight to the satellite, such as the roof, a window, etc. Thus, thepresent invention reduces initial investment costs for consumers andimproves aesthetics, which permits a DTH system that can effectivelycompete with existing CATV systems in urban areas yet also accommodatesusers in rural areas where CATV is not feasible. Thus, the presentsystem combines the advantages of DTH systems, i.e., accessibility torural users, with the advantages of CATV systems, relatively low costinstallation for urban areas. In fact, the system of the presentinvention costs less to install than a CATV system, if the satellitesare already in existence.

Ku-Band Satellite Communication System and Antenna

The same general technique is possible for a Ku-Band satellitecommunication system and antenna. Generally, the problem is similar tothe problem in the C-Band system. Ku-Band frequencies (15.35-17.25 GHz)are used for direct broadcast television. Due to the frequenciesinvolved some differences exist in the orbital spacing of the satellitesand the allowable FCC power limitations. The FCC power limits are higherat Ku-Band than C-Band, however, losses due to rain absorption andthermal noise are higher at Ku-Band frequencies. Therefore, to use asmaller antenna at Ku-Band than what is in current use (about two tofive feet in diameter) normally would require higher radiated power.This is not possible due to the FCC limitations. However, equivalentsize savings on Ku-Band antennas are possible with the combination ofthe video compression, spectral shaping and antenna design techniquesdiscussed above, when tailored for the Ku-Band environment.

Essentially for the same constellation of satellites discussed above,spaced at ±2° intervals, the antenna dimensions reduce by the ratio ofthe respective wavelengths. The gaps in the antenna remain in the sameproportional locations as for the C-Band system. For example, to modifythe antenna from C-Band to Ku-Band, the scaling ratio becomes: ##EQU3##

Thus, the Ku-Band antenna can be scaled down directly from the C-Bandversion by a factor of about 3.23. Since the C-Band antenna has an areaapproximately equivalent to a three foot diameter dish, the K-Bandantenna has an area approximately equivalent to a one foot diameter dishor less, with gaps in the same proportions as the C-Band system. Forexample, let "x" denote the position from the center of the antennawhere the gaps are located, then x/3.23 denotes the placement of thegaps in the Ku-Band version of the antenna of the present invention.Current Ku-Band parabolic dish antennas are about 3 feet in diameter,hence the present invention permits a significant reduction in antennasfor this frequency band as well.

L-,S- and Ka-Band Communication Systems and Antennas

The same technique is also possible to reduce the receiving antenna sizefor other frequency bands, such as L-Band (0.390-1.550 GHz), S-Band(1.55-5.20 GHz) and Ka-Band (33-36 GHz). To reduce the antenna sizerequires a reduction in the amount of data per unit bandwidth, which issolved by data compression techniques. Where television signals areinvolved, video data compression techniques permit a significantreduction in data, approximately 90% (i.e., the compressed digitalsignals are about 1/10 the information rate of the uncompressedsignals). This by itself is not sufficient to significantly decrease theantenna size with existing satellite configurations. When combined witha modulation technique that reduces the power flux density, providessome gain, and allows an increase in the transmission power, thereceiving antenna can be significantly reduced in size.

The receiving antenna must also be designed according to the presentinvention to permit the receiving antenna to operate in the transmissionfootprint of multiple satellites in the constellation, yet stilldiscriminate between the satellite of interest and those adjacent to thesatellite of interest. The same technique described herein will work inthe S-, L- and Ka-bands. The only special considerations for operatingat S-, L- and Ka-bands are the shaping of the antenna to achieve therequired nulls in the antenna patterns, the specific spacing betweensatellites in orbit and the satellite power required including theeffect of compression and also rain absorption. A unique combination ofbeam shape, antenna shape, modulation shape and the number of televisionchannels per transponder is appropriate for each band following theprocedure illustrated by the basic C-Band system description.

As before, for the same constellation of satellites discussed above,spaced at ±2° intervals, the antenna dimensions are reduced by the ratioof the respective wavelengths. The gaps in the antenna remain in thesame proportional locations as for the C-Band system. For example, tomodify the antenna from C-Band to L-Band, the scaling ratio becomes:##EQU4##

Thus, the L-Band antenna can be scaled up directly from the C-Bandversion by a factor of about 5.2. Since the C-Band antenna has an areaapproximately equivalent to a three foot diameter dish, the L-Bandantenna has an area approximately equivalent to a 15.6 foot diameterdish, with gaps in the same proportions as the C-Band system. Forexample, let "x" denote the position from the center of the antennawhere the gaps are located, then 5.2x denotes the placement of the gapsin the L-Band version of the antenna of the present invention. ExistingL-Band parabolic dish antennas are approximately three times indiameter, hence the present invention permits a significant reduction inthe receiving antenna for L-Band implementations as well.

For example, to modify the antenna from C-Band to S-Band, the scalingratio becomes: ##EQU5##

Thus, the S-Band antenna can be scaled up directly from the C-Bandversion by a factor of about 1.5. As a result, since the C-Band antennahas an area approximately equivalent to a three foot diameter dish, theS-Band antenna has an area approximately equivalent to a 4.5 footdiameter dish, with gaps in the same proportions as the C-Band system.For example, let "x" denote the position from the center of the antennawhere the gaps are located, then 1.5x denotes the placement of the gapsin the S-Band version of the antenna of the present invention. ExistingS-Band parabolic dish antennas are approximately three times indiameter, hence the present invention permits a significant reduction inthe receiving antenna for S-Band implementations as well.

For example, to modify the antenna from C-Band to Ka-Band, the scalingratio becomes: ##EQU6##

Thus, the Ka-Band antenna can be scaled down directly from the C-Bandversion by a factor of about 6.8. As a result, since the C-Band antennahas an area approximately equivalent to a three foot diameter dish, theKa-Band antenna has an area approximately equivalent to a 0.44 footdiameter dish, with gaps in the same proportions as the C-Band system.For example, let "x" denote the position from the center of the antennawhere the gaps are located, then x/6.8 denotes the placement of the gapsin the Ka-Band version of the antenna of the present invention. ExistingKa-Band parabolic dish antennas are approximately three times indiameter, hence the present invention permits a significant reduction inthe receiving antenna for Ka-Band implementations as well.

In using the design technique of the present invention, if the area ismore or less than that desired, if for example, due to satellite powerlimitations or rain losses, then the gaps in the East-West dimensionscan be kept as before, but the widths in the North-South dimensions canbe scaled to achieve the desired area. For example, the North-Southdimensions could be doubled to achieve twice the area without changingthe locations of the antenna nulls in the East-West orbit positions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an embodiment of the antenna of the present invention foruse in a C-Band satellite communication system operating according tothe method of the present invention.

FIG. 2 depicts a typical antenna pattern of a normal three foot dishantenna receiving signals from the current constellation of televisionbroadcast satellites operating in the C-Band range.

FIG. 3 depicts the distance travelled by all rays from the feed-horn tothe reflector to the target satellite, when the direction to the targetsatellite lies along the main axis of a parabolic antenna.

FIG. 4 depicts the distance travelled by all rays from the feed-horn tothe reflector to the target satellite, when the direction to the targetsatellite lies is not along the main axis of a parabolic antenna, butrather lies along a path offset from the main axis.

FIG. 5 depicts the signal strength of the received signal on the surfaceof the antenna of an embodiment of the C-Band antenna of the presentinvention, as seen from a top view of the antenna.

FIG. 6 shows the projection of energy arriving at one embodiment of theC-Band antenna of the present invention from the satellite 2.24° fromthe central satellite.

FIG. 7 shows the projection of energy arriving at one embodiment of theC-Band antenna of the present invention from the satellite 4.48° fromthe central satellite.

FIG. 8 shows the projection of energy arriving at one embodiment of theC-Band antenna of the present invention from the satellite 6.72° fromthe central satellite.

FIG. 9 depicts the received signal strength from the off axis satellitesof one embodiment of the C-Band antenna of the present invention.

FIG. 10 depicts an embodiment of the satellite portion of the presentinvention operating in C-Band.

FIG. 11 depicts an embodiment of the ground station portion of thepresent invention FIG. 12 depicts a cross-section of the C-Band antennaof the present invention and the corresponding received signal strength.

FIG. 13 depicts a general view of the C-Band antenna of the presentinvention.

FIG. 14 is a graph of the cross-section of the dish along F'-E-F (orG'E-G) in FIG. 13, in side view, plan view and right view.

FIG. 15 depicts a satellite receiving the signal of the presentinvention and retransmitting the signal back to earth.

DETAILED DESCRIPTION

Description of the Antenna for the C-Band System

FIG. 1 depicts an embodiment of the receiving antenna of the presentinvention for use in the C-Band satellite communication system. Asevident in FIG. 1, the antenna comprises a main reflector 1, and twoside reflectors 2 and 3, as well as an antenna feed 4. The mainreflector 1 and both side reflectors 2 and 3 have parabolic surfaces.The radius of the embodiment of the antenna shown in FIG. 1 from thefeed 4 to the main reflector surface 1 is F₁, which is about 20.0inches. The radius from the feed 4 to the side reflectors is F₂, whichis about 28.8 inches. The antenna has a fresnel step equal to F₁ -F₂,i.e., about 8.8 inches. The length of the antenna from the outside edgeof one side reflector 2 to the other 3 is 57.5 inches. The width orhorizontal dimension of the side reflectors, 2 and 3, is 19.2 inches,and the width or horizontal dimension of the main reflector is 13.3inches. The vertical dimension of the main reflector is about 10.55inches, while the vertical dimension of the side reflectors is about5.48 inches.

The antenna of the present invention uses a spill over baffle to preventthermal energy from the ground behind the antenna from reaching thefeedhorn. As seen in FIG. 1, the spill over baffle 6 is located in theouter edge of the outer reflectors.

The aperture synthesis for the C-Band antenna of the present inventionis designed to provide normal gain for a satellite at one position inthe synchronous orbit and provide low-gain nulls for satellites at ±2°,±40°, ±6° and ±8° away in synchronous orbit. The depth of the nulls canvary, but must be at least 10 dB to prevent interference. Thus, theantenna is designed to receive from only one of the satellites in aconstellation at a time, while simultaneously inhibiting reception fromthe remaining satellites in the constellation, especially those directlyadjacent to the targeted satellite. The reason for inhibiting receptionfrom these satellites is that they are the single largest source ofpotential interference since they are transmitting at nearly the sameradio frequencies, but the signals contain different programs than thesignal from the targeted satellite. These interfering signals willseriously distort the received signal, and prevent proper decodingunless they are suppressed by the antenna pattern.

In the C-Band version of the present invention, the actual position ofthe required nulls are in fact a little wider than the ±2° spacingbecause the antenna on the surface of the earth is closer to theconstellation of satellites than the earth's center, as evident in FIG.2. FIG. 2 depicts the typical antenna pattern of a three foot receivingantenna receiving signals from the current constellation of televisionbroadcast satellites operating in the C-Band range. The arrows in thefigure represent interfering satellites 11, 13, 15, 19, 21 and 23. Thetarget satellite 17 is centered at the maximum gain of the receivingantenna.

The 3 db cutoff frequency of the receiving antenna is about 5.68° fromcenter. A diameter of 36" equals about 91 centimeters. The wavelength λof a representative signal in the C-Band, i.e., about 4 GHz, isdetermined by: ##EQU7##

The three-dB cutoff angle, α, is determined typically by the followingformula. Thus, at 4 GHz a becomes: ##EQU8##

The difference from the ±2° spacing varies from a maximum of ±2.35°,when the satellites are above the same longitude as the ground antenna,to ±2.11° when the satellites are 60° east or west of that position.These variances can be accounted for by designing the gaps to be at themean of these values, and providing sufficient depth in the null toaccount for when the receiving antenna is at the extremes.

In analyzing the antenna performance, it is most useful to treat theproblem as a ray tracing problem. The antenna gain can be found bytracing rays from the antenna feed horn 4, or central power collector,to the surface of the reflector 1 and from there out to the fardistance, or "infinity" in a particular direction. When the directionlies along the main axis of a parabolic antenna, the distance traveledby all rays from feed-horn to reflector to the distant point is thesame, as shown in FIG. 3. All increments of power traveling those pathswill arrive with the same delay, since 1_(w) =1_(o) =1_(e). Therefore,the field in that direction will receive all increments of energy inphase with each other reinforcing each other for maximum gain.

Conversely, for a direction some degrees off the main axis (as shown inFIG. 4), the energy reflected from different parts of the parabolicreflector 1 travel paths of different lengths. Energy from the reflectorside 1_(w) that is closer to the direction of the main axis (the nearside) travels a shorter path than energy from the center of thereflector, 1_(o), hence 1_(w) <1_(o) <1_(e). As a result, the energyfrom the near side arrives earlier in phase than the energy from thecenter; and the energy from the far side arrives later in phase than theenergy from the center. When energy from all of the reflector iscombined at the distance, some increments add and some subtract from thewhole. In aggregate, the sum totals less than the sum of the energy inthe main axis of the antenna. When the distance difference from thecenter of reflector to the edge of reflector reaches one half of theradio frequency wavelength, α, the energy from the edges directlysubtracts from the energy from the center. The diameter of the antennaat this point is given by the relationship: ##EQU9##

With a normal reflector feed design, at an angle twice φ_(1/2), energyfrom the edges is delayed by one whole wave length and is back in phasewith energy from the center. This will cause the energy at 2×φ_(1/2) tobe reinforced again causing a minor peak in the antenna pattern.

The values where the actual nulls in the patterns and the subpeaks occurdepend precisely on the shape of the feed horn pattern illuminating thereflector, the shape of the reflector and the blockage of any energy bystructures, such as the feed horn and its supports. For a typicalantenna design, the diameter required to reduce the gain to reasonableprotectionlevels for ±2° satellite spacing at C-Band, f=4 GHz., λ=0.075m, is about 8.5 feet. In the antenna of the present invention, eventhough digital television compression and spectral shaping would allowan antenna as small as 3 feet in diameter, the interference ofsatellites at ±2.24°, ±4.48°, etc. preclude the standard small dish.

The present invention solves this problem by using sections of parabolaswith areas blocked out to control the phases of energy reaching thedirections at ±2.24°, ±4.48°, etc. in such a way as to cause nulls, orlarge attenuations, at precisely these positions in the satellite orbit.The basic parabola is pointed at the desired satellite of interest(referred to as the targeted satellite) and all field components indirections towards the interfering satellites add up with differentphase angles to cause the precise cancellations.

FIG. 1 depicts one embodiment of the present invention. The top view ofthe antenna shows that three sections of the normal parabolic surfacehave been retained, a central section and two side sections in theeast-west directions parallel with the earth's equator. The sectionwidths in the north-south direction can be adjusted to increase ordecrease the amount of energy in any east-west location. For example,the outer contour of the antenna may have an irregular shape rather thana smooth curve to add and subtract area in the antenna to add orsubtract the energy reaching the antenna as desired. The north-southdimension can be reduced to zero, i.e., by placing a gap betweensections, to accomplish the desired nulls, or it can be widened toincrease energy in desired directions.

FIG. 6 shows the projection of energy arriving at the satellite 2.24°from the central satellite. The reference phase is that of energyarriving from the center of the reflecting surface. What is plotted isthe cosine of the phase angle between this central energy and energyfrom off axis positions. As energy comes from further away from thecenter, the projection decreases, goes through zero and becomesnegative. A gap 7 exists between the main reflector 1 and the two sidereflectors, 2 and 3.

FIG. 7 shows the same projection but this time for energy arriving at±4.48° from the central satellite. The same behavior is seen but now thezero appears twice as close to the center.

FIG. 8 shows the same for satellites at ±6.72°. Similar plots are foundat ±8.96°, etc.

To find the field strength at 2.24°, the shaded area of FIG. 6 would beintegrated over the reflecting surface. More precisely, the physicalarea of each reflector element should be further weighted by the gain ofthe feed horn antenna in the direction of the reflector elements.

In the example of FIG. 6, the reflecting area and gap are selected toillustrate the principle. The gap size and location have been selectedto cause energy from the positive central section to cancel withnegative phases for the two edge sections. This causes a null at ±2.24°off the axis from the central satellite.

In this particular example, the energy at ±4.48°, shown in FIG. 7, willnot precisely cancel as can be found by integrating FIG. 7 over theaperture and gap. However, to improve on cancellation at ±4.48°, areacan be added or subtracted at any point by increasing or decreasing thenorth-south width of the main reflector. In FIGS. 6-8 this direction isinto the paper. If area is added in the region where the ±2.24° curve,FIG. 6, goes through zero, then there will be no change to the ±2.24°cancellation while the ±4.48° cancellation will be improved.

In a similar way, areas of the antennas where the ±4.48° curve goesthrough zero can be increased or decreased in width to refine the nullsat ±2.24° and ±6.72° without affecting the null at ±4.48°. Since onlyfour sets of nulls need to be canceled before the pattern will remainbelow the required level, the problem is underconstrained, i.e., thereare many different fine adjustments of the widths in the North-Southdimension that can be made to cancel the signals in the ±2.24°, ±4.48°,±6.72° and ±8.90° locations.

Because the antenna's gain beyond 8° is systematically below therequired levels, the cancellation at ±2.24°, ±4.48°, ±6.72° and ±8.96°need only be balanced using the procedure of the present invention.There are many more degrees of freedom in the design than necessary tocreate the required eight nulls in the east-west pattern of the antenna.A number of combinations of feed horn gain, gap sizes and north-southwidth choices exist that create the required notches. The embodimentillustrated in the drawings utilizes apertures that are rectangular inoutline as projected in the direction of the satellite, a central areathat is blocked by the feedhorn, and avoidance of gaps between the mainand side reflectors as seen from the feed point.

FIG. 5 illustrates the antenna pattern resulting from the antennadepicted in FIG. 1. The numbers in the top view in FIG. 5 represent thefield strength of the feed horn on the antenna surface. The gap andnorth-south dimensions have been chosen to accomplish the desiredcancellation. As shown in FIG. 5, the 0's in the central sectionrepresent feed horn blockage; B represents the width of the outerreflector; B, represents the width of the inner reflector; and W is thewidth of the antenna. Only one half of the antenna is shown. The numbersin FIG. 5 depict the signal strength on the antenna at the particularcoordinates. For example, 25 mV is the electric field strength of theantenna at 28 inches in the East-West direction and about 5 inches inthe North-South Direction. The dots represent places where the signalstrength is effectively zero. FIG. 9 shows the gain of the antenna atthe different angles showing the desired nulls in the desired positions.Thus, FIG. 9 depicts the off-axis performance of the antenna.

FIG. 1 also shows a second innovation of the antenna of the presentinvention. The full gap between center and side sections allows the useof what is termed a Fresnel lens improvement. As long as the antennareflecting surface is parabolic, the gain will be achieved in theprincipal direction. One feed can be used with different parabolicsurfaces. Near the center, parabolas with short focal length are used,near the edge parabolic surfaces with large focal lengths are used. Aslong as each parabola differs from a reference focal length by integermultiples of 1/2 wavelength, the energy in the principal beam will alladd in phase as if they were from a single parabolic surface. Thisallows approximately the same performance with an antenna that isphysically thinner. The steps between parabolas are made abruptly,causing a bit of loss from fringing effects, but also giving somemechanical strength improvements in some designs. In this embodiment,the fresnel step is F₁ -F₂.

In the antenna of the present invention, an advantageous embodiment ismade by having the central section constructed with ashorter-focal-length parabola than the end sections. This embodiment hasthree advantages. First, it makes the structure smaller and stronger.Second, it improves the antenna efficiency by having the gap requiredfor cancellation take up the area that would be in shadow as seen by thefeed horn. Third, it makes the feed horn pattern easier to realize,since the desired central reflector is smaller in the north-southdimension than the edge reflectors. The ideal feed would normally have adumbbell pattern, pinched in the middle and wider at the sides. Movingthe central reflector near to the feed increases the ideal beam width ofthe feed in the center section, making it more nearly oval andrealizable with a more standard feed horn.

The feed horn shown in the embodiment of FIG. 1 is 2.25 inches in theEast-West dimension and 8 inches in the North-South dimension. The feedhorn pattern that results from this configuration is elliptical, whichis wide in the East-West dimension and narrow in the North-Southdimension. One other possible design for the feed horn would be to use aconventional rectangular pyramidal horn. A feed horn design with anelliptical mouth rather than a rectangular mouth would also suffice.

The Fresnel step between center and outside parts of the antenna thusincreases the efficiency of the feed horn without compromising theactual area of the antenna of the performance at the desired nullpoints. While the present embodiment depicts a particular fresnel stepin which the focal length increases from the main reflector to the sidereflectors, a number of different step options are possible inalternative embodiments.

Thus, the antenna of the present invention is designed to shape the feedpattern and the reflector area to create nulls at the interferingsatellite points. The technique to do this has been described above.While one realization has been illustrated and described in detail, manyvariations are possible.

One specific addition to the general approach is to use differentfocal-length parabolas on physically separated sections. Thisconfiguration can improve the feed efficiency as well as the mechanicaldesign.

FIG. 12 depicts a cross-section of one half of the antenna. FL=focallength. The bottom part of FIG. 12 indicates the aperture distributionof the antenna of the present invention. E(n) represents the totalsignal strength of the antenna at a given East-West location for theentire North-South dimension. Thus, E(n) is the integral of the signalstrength at a particular East-West location as the integral runs along astrip from one edge to the other in the North-South dimension. At 4 GHz,the center reflector is raised 3 wavelengths with respect to the outerreflector.

The physical design of the antenna will be described next. The antennaconsists of four parts: a central section 1, two wings 2 and 3, and aframe 8, as depicted in FIG. 13. FIG. 13 is a general view of theantenna. The shape of the frame, which is rendered schematically, maytake any suitable form.

The central dish is a 13.28"×13.28" square segment of a paraboloid offocal length 19.95 inches (H-H'-J'-J in FIG. 13), centrally situatedaround the vertex of a paraboloid.

The height of the surface above the vertex at radial distance r ininches measured in the level plane is expressible as r² /79.8. Forexample, at the center of each square side the height is6.64×6.64/79.8=0.553 inches above the vertex. At the corners the heightis 1.106 inches.

Table I gives the surface heights above a grid of points spaced one inchin both directions in the level plane. Only one quarter of the square istabulated. The remaining parts of the square can be determined from thesymmetry of the antenna.

FIG. 14 is a graph of the cross-section of the dish along F'-E-F (orG'E-G) in FIG. 13. The drawing in the upper left depicts a side view ofthe antenna; the drawing in the lower left depicts a plan view of theantenna; and the drawing in the lower right depicts a right view of theantenna. The graph depicts the height of the center section of theantenna above E, which is the center point of the center section.

                  TABLE I    ______________________________________    Surface heights for central dish    edge    0     1       2   3     4   5     6    6.64    ______________________________________            E                                      F    0        0     1       5  11    20  31    45   55    1        1     3       6  13    21  33    46   56    2        5     6      10  16    25  36    50   60    3       11    13      16  23    31  44    56   66    4       20    21      25  31    48  51    65   75    5       31    33      36  43    51  63    76   87    6       45    46      50  56    65  76    90   100    6.64    55    56      60  66    75  87    100  110            G'                                     H'    ______________________________________

Each wing is a rectangular segment of a paraboloid of focal length 28.8inches (C-D-D'-C) in FIG. 13. The inner edge CC' is situated at adistance 14.0 inches from the axis of the central dish, measured in thelevel plane. With respect to the (x,y,z) coordinate system indicated inFIG. 17, the positions of representative points are as given in TableII.

The height of the surface of the wings at radial distance r in inchesmeasured in the level plane from the axis of the central square dish, isexpressible as r² /115.2. For example, at point A in FIG. 17, which is14.01 inches from E, measured horizontally, the height is14.01×14.01/115.2 =1.704 inches (compare line 1 of Table II).

                  TABLE II    ______________________________________    Coordinates of representative points    Point    x             y      z    ______________________________________    A        14.01         0      1.7    B        28.76         0      7.18    C        14.01         9.59   2.5    D        28.76         9.59   7.98    E        0             0      8.95    F        6.64          0      9.4    G        0             6.64   9.94    H        6.64          6.64   9.95    ______________________________________

Table III gives the depth of the wing relative to the plane passingthrough the corners C, D, D', and C'. Only one half of the rectangle istabulated.

                                      TABLE III    __________________________________________________________________________    Wing surface heights in hundredths of an inch.    Edge       1 2  3  4  5  6  7  8  9  10 11 12 13 14   15 15.74    __________________________________________________________________________    A                                                B    85 97         108            117               124                  129                     133                        135                           136                              135                                 132                                    128                                       122                                          115                                             106                                                95                                                  86 C/L                                                     0    85 97         107            116               123                  128                     132                        134                           135                              134                                 131                                    127                                       121                                          114                                             105                                                94                                                  85 1    82 94         104            113               120                  126                     129                        132                           132                              131                                 129                                    124                                       118                                          111                                             102                                                91                                                  82 2    77 89         100            108               115                  121                     125                        127                           128                              127                                 124                                    120                                       114                                          106                                             97 88                                                  77 3    71 83         93 102               109                  114                     118                        121                           121                              120                                 118                                    113                                       107                                          100                                             91 88                                                  71 4    62 74         85 93 101                  106                     110                        112                           113                              112                                 109                                    105                                       99 92 82 72                                                  63 5    52 64         75 83 90 96 100                        102                           103                              102                                 99 95 89 81 72 61                                                  52 6    48 52         62 71 78 84 88 90 91 30 27 83 77 69 60 49                                                  40 7    26 38         49 57 64 70 74 76 77 76 73 69 63 55 46 35                                                  27 8    10 22         33 42 49 54 58 60 61 68 57 53 47 40 31 20                                                  11 9     0 12         23 31 39 44 48 50 51 50 47 43 37 29 20 10                                                  0  9.53    C'                                            D'    __________________________________________________________________________

The surface of the model is electrically conducting and generallysmooth. The present embodiment does not differ from the tabulated valuesby more than 0.15 in r.m.s.

A frame of electrically nonconducting material holds the dish and twowings in position relative to each other as in Table III, within anaccuracy of ±0.05 inches. The frame should be rigid, robust, andportable. It is possible to affix other elements, especially a feedsupport bracket or brackets and electrical cables.

While the above description referred to placing narrow nulls in specificlocations, the present invention would also operate if the nulls werereplaced by broad attenuation at these same locations. All that isrequired is to adequately reduce the signal strength below the thresholdat which interfering signals would impair reception. While thisthreshold varies with each implementation, attenuating the signals by 10dB should be sufficient.

Furthermore, while the above description created these nulls by placinggaps between the main reflector and the two side reflectors, anyeffective gap would also suffice. An effective gap is defined herein asa place where the area in the antenna is reduced significantly but notto zero. Thus, a "neck" could exist between the main reflector and eachof the two side reflectors, but not a gap. Such a design may haveparticular advantageous properties, such as ease of fabrication.

In addition, the above antenna design described a symmetrical antenna.An asymmetrical antenna would also suffice, as long as the signalstrength of the interfering signals was reduced below the abovethreshold.

Finally, the antenna of the present invention would work in applicationswhere the underlying data was something other than television. Theantenna will apply to any system in which the user desires to reduce thesize of the antenna to a point at which its beam width no longer coversonly one satellite, but rather is now receiving interfering signals fromsatellites near the satellite of interest as well as the desired signal.

Finally, while electronic phase cancellation techniques are known, theyare very expensive due to the complex equipment involved. The presentinvention performs its aperture synthesis without the benefit of complexelectronics.

Description of the Spectral Shaping Technique

The present invention uses a bandwidth spreading technique to reduce thepower density below the FCC thresholds for each of the systems. Thistechnique also reduces the effect of interfering signals on the receivedsignal.

The present invention uses a Shaped Frequency Shift Key (SFSK)modulation scheme to spread the energy of the transponder smoothly overthe bandwidth of the satellite transponder, which in the case of theC-Band system spreads the signal over the 30 MHz of the current C-Bandsatellite transponder. Frequency Shift Keying (FSK) is a commonly knownmodulation technique. Minimal Shift Keying (MSK) is an FSK signalshifted in such a way as to minimize the frequency spreading. By "SFSK"we mean any of many shapes of frequency versus time patterns that willoccupy the wider bandwidth without losing power efficiency. By spreadingthe bandwidth from 5 MHz to 30 MHz using the SFSK modulation technique,the power density is reduced below the FCC limitation.

The SFSK signals themselves have a "coding gain" approximately equal tothe transponder bandwidth divided by the data rate, a protectionachieved for any power efficient modulation scheme. This amounts to aprotection factor of between three and ten depending on the number oftelevision signals of the present invention in one satellitetransponder.

Three different SFSK modulation shapes are available, depending onwhether the transponder of the present invention uses one, two or threechannels, which depends on the power from the transponder. A transponderradiating its signal at 30-31 dB EIRP transmits one television channelusing a particular SFSK modulation; a transponder radiating its signalat 31-33 dB EIRP transmits two channels using a different modulation;and a transponder radiating its signal at 35 dB or greater EIRP cantransmit three channels using a third modulation.

One possible embodiment of this aspect of the present invention usesManchester Encoding for the three channel implementation. For example,if the information data rate is 5 megabits per second (MBPS) perchannel, the total information data rate becomes 15 MBPS. By coding theinformation bits into two transmitted bits using Manchester Encoding,the 15 MBPS signal will be transformed into a 30 MBPS signal, whicheasily occupies the 30 MHz bandwidth of the transponder.

For the two channel implementation, i.e., a 10 MBPS signal must betransformed into a 30 MBPS signal. This can be accomplished by usingthree data bits per information bit, i.e., triple redundancy. As before,the resulting 30 MBPS signal can easily occupy the 30 MHz bandwidthavailable on the satellite transponder.

For the one channel implementation, i.e., a 5 MBPS signal would betransformed into a 30 MBPS signal, which can be accomplished using a 6data bits per information bit. The modulation desired can beaccomplished using digital bit expansion as above followed by spectralshaping, or alternatively, by any of a number of shaping filters on thetransmitter and matched filters on the receiver.

In addition to modifying the spectrum to occupy the full bandwidthavailable as described above, the present invention modifies the numberof channels depending upon the transponder EIRP available on the chosensatellite. For example, if the satellite transponder has only 31 dB EIRPavailable, then the system of the present invention will send only onechannel via that satellite. Additional protection is thereforeautomatically provided by the resulting coding gain, e.g., 30 MHz/5MBPS, which is a factor of six. For example, if the satellitetransponder has only 33 dB EIRP available, then the system of thepresent invention will send two channels via that satellite. Additionalprotection is therefore automatically provided by the resulting codinggain, e.g., 30 MHz/10 MBPS, which is a factor of three. Finally, if thesatellite transponder has 35 dB EIRP or more available, then the systemof the present invention will send three channels via that satellite. Noadditional protection is necessary.

The adjustment of number of channels for a given satellite EIRPequalizes the interference performance of the system. Normally thesatellite radiating a stronger signal, such as 35 dB EIRP, would providethree times the interference to a weaker signal at 30 dB, for example,from an adjacent satellite. As a result, the signal from the higherpower satellite would require one-third the coding gain to protect itfrom the signal from the weaker satellite. Using only one televisionchannel on the satellite radiating the weaker signal automaticallyprovides the required improvement in protection for the weakersatellite. The choice of SFSK modulation type and channels pertransponder is deliberately made to equalize the protection needed bythe antenna pattern and the television demodulator no matter which ofthe satellites is being received in a constellation of satellites ofunequal power.

Description of the Video Compression Technique

The present invention incorporates existing data compression techniques.All that is required is a data compression algorithm that reduces thedata by a factor of about ten. The embodiment of the present inventionuses a commercially available product from Scientific Atlanta to providethe required video data compression. This same product will suffice inall embodiments, i.e., the C-, S-, L-, Ku- and Ka-Band systems.

Description of the C-Band and Ku-Band Communication Systems

The basic embodiment of a system for transmitting the signal of thepresent invention is depicted in FIG. 10. The general block diagram willnot vary when the system is changed to a different band, such as Ku-,Ka-, L- or S-Band. The only change occurs in the satellite transmitter117 and the antenna 118, which now radiate the signal to the satelliteat a different RF frequency.

The system operates as follows. FIG. 10 depicts the ground transmitterof the present invention. The video signal 110 is converted by an analogto digital converter 111 into a digital signal 112. The digital signal112 is converted into a compressed digital signal 114 by the datacompressor 113, which has been described above. The compressed digitalsignal 114 is modulated by the SFSK modulator 115 into a wideband analogSFSK uplink signal 116, using the modulation technique described above.The ground station transmitter 117 transmits this wideband analog SFSKsignal using the ground station antenna 118, which radiates the RFsignal 119 to the satellite. A satellite transponder 130, which isdepicted in FIG. 15, receives the incoming wideband analog SFSK signal119 with antenna 131, passes it to receiver 132 which outputs the SFSKsignal to frequency translator 133, which shifts the signal in frequencyto a desired downlink frequency, such as a C-Band frequency, forexample, which is different than the uplink frequency to preventinterference. The transmitter 134 outputs the wideband analog SFSKsignal at that frequency and radiates the RF signal 136 towards theearth.

The RF signal constitutes a broadband signal centered at the carrierfrequency of the satellite transponder, which in the C-Band system isapproximately 4 GHz. The details of the link are set forth below.

FIG. 11 depicts the ground portion of an embodiment of the presentinvention. The antenna 120, which is of the type described above,receives the RF signal 136 transmitted from the satellite, along withinterfering signals and noise. The antenna 120 outputs the receivedsignal to an SFSK demodulator 121, which converts the received signalinto a compressed digital signal 122 that approximates the compresseddigital signal 114 from FIG. 10. The SFSK demodulator outputs thiscompressed digital signal 122 to a data restorer or decompressor 123,which converts the compressed digital signal 122 into a digital signalthat approximates the digital signal 112 from FIG. 10. The datadecompressor passes the digital signal 124 to a digital to analogconverter 125, which converts the digital signal 124 to a video signal126 that resembles the video signal 110 in FIG. 10. Thus, the systemcommunicates the video signal from the ground transmitter, shown in FIG.10, via a satellite transponder 130 to a user on the ground, who is ableto employ an antenna with a receiving area equivalent to a dish having athree foot or less diameter, yet which satellite does not violate FCCregulations regarding transmitted power. Additional control and datatransmission signals of lower overall data rate can be added attransmitter and receiver to manage billing and to deliver additionalinformation to the user. Typically, the data compression and expansioninclude encryption techniques to protect proprietary materials.Furthermore, error correction and detection techniques may also beemployed without corrupting the present invention. The exact RF signallevels will be set forth below in the link equations.

The physical equations that define the relations between the transmittedsatellite radio power and the size of the dish antennas are usuallycalled the "link equations." In normal algebraic form they define theratio of signal power received, P_(r), to noise power received, P_(n).The signal power received is determined by: ##EQU10## where: P_(s)=transmitted satellite power;

G_(s) =satellite antenna gain, the ability to focus the power on justthe country being served;

ηA_(r) =the effective area of the receive antenna;

π=3.1415927;

R=the distance from satellites to ground station, typically;

A_(b) =absorption factor due to rain and atmosphere.

The noise power received by the antenna is a function of temperature,and can be determined by:

    P.sub.n =kT.sub.r B                                        (11)

where:

k=1.38×10-23, Boltzman's constant, a physical constant relatingtemperature of "black body" radiation;

T_(r) =effective radiation temperature of the receive station;

B=the bandwidth of the signal being received. Television in its variousforms has a bandwidth that varies from 4.7 MHz for normal broadcast, to30 MHz for satellite FM television, and from to 1 MHz to 8 MHz forcompressed digital television.

The required performance is given by a minimum ratio (C/N) of receivedsignal power P_(r), which is determined from equation (10), to noiseP_(n), which is determined from equation (11). Thus, C/N becomes:##EQU11##

The required ratio (C/N) is determined by the required televisiontransmission mode and varies from normal broadcast television, toSatellite FM television relay, and to the new digital televisionbroadcast. Since the choice of transmission mode defines the bandwidth Band the required (C/N) at the same time, these two parameters areusually grouped together: ##EQU12##

The required satellite power (P_(s)) can be defined in terms of theother system choices by factoring of terms in equation (12). ##EQU13##

Equation (14) contains the information relevant to the comparison ofSatellite TV services offered in the two frequency bands, C-Band andKu-Band. The satellite power, P_(s), is the primary space segment costfactor because the scarce solar cell power has to be divided among thepowers required for each transponder, which determines how much eachchannel shares in the total satellite cost.

The two factors shown in equation (13) are determined solely by thechoice of the TV modulation type to be used. The most relevant types arenormal broadcast TV, Satellite FM-TV the type currently used in C-BandSatellites, and compressed video. Table IV below lists the values forthese three types of TV transmission.

                  TABLE IV    ______________________________________    Modulation Parameters, B•C/N    TV Type     B           C/N    B•C/N    ______________________________________    Broadcast TV                4.6 MHz     3000   1380 × 10.sup.7    Satellite FM-TV                 30 MHz       8     24 × 10.sup.7    Compressed    5 MHz       4      2.0 × 10.sup.7    Video    ______________________________________

Normal broadcast television requires over 20 times greater power fromthe satellite than FM television. Even though some information agencieshave proposed transmitting directly from satellites to home televisionsets, the need for twenty times the satellite power has proven to beimpractical.

The standard satellite FM television has been used for years as thebasic-technique for both C-Band and Ku-Band satellites. Until justrecently it was the best available to minimize required satellite power.The advent of the new digital signal processors (DSPs) has madecompressed digital television practical. The compression reduces theB·C/N parameter by a factor of 10, which means that without any otherchange in the system the satellite power or the antenna area can bereduced by a factor of 10 simply by changing to the new televisionsystem. The performance improvement applies to both Ku- and C-Bands, aswell as S-, L- and Ka-Bands.

The range to the geosynchronous satellites, R, is typically 40,000 Km.As a result, the factor 4π² becomes 2×10¹⁶ m², which is the same forboth Ku-Band and C-Band.

The gain of the satellite, G_(s), depends entirely on the area of thecountry to be covered. For normal coverage of the U.S., a gain ofapproximately 25 dB, which is 300 in algebraic terms, is achievable. Thesatellite's gain is limited to 300 by the area of geographic coverage,whether C-Band or Ku-Band is used. To achieve this gain Ku-band must usea smaller satellite antenna, which results in some weight savings on thesatellites, but not enough to much affect the satellite cost.

The total power in the satellites is also proportional to the effectivearea of the receive station antenna, ηA_(r). For an antenna with anefficiency of 60%, the effective area is ##EQU14## A directly relatedparameter is the antenna gain given by the equation: ##EQU15##

Table V gives the effective areas of several possible embodiment ofantennas. The effective area is also independent of Ku-band or C-bandfrequency choice. The related gain is dependent on the frequency bandchosen.

                  TABLE V    ______________________________________    Effective Antenna Areas at 60% Efficiency    Antenna Diameter    (Inches)    (meters)  60% Area Gain at 4 GHz    ______________________________________    18          0.46      0.1 m.sup.2                                   23.5 dB    36          0.92      0.4 m.sup.2                                   29.5 dB    72          1.8       1.6 m.sup.2                                   35.5 dB    ______________________________________

The remaining two parameters, T_(r) and A_(b), are strongly dependent onthe choice of C-Band or Ku-Band frequency. C-Band frequencies are littleeffected by rain while Ku-Band frequencies, which are a lot closer tothe rain frequencies, are much worse. A lot of statistical data has beengathered to determine the margins required, the values used below aremidway between extremes. The rain margin at C-Band frequencies istypically 0.8 dB, or 1.20 in algebraic terms. The rain margin for thesame rates of rainfall at Ku-Band are 8 dB, which is a factor of 6.3 inalgebraic terms. These absorption factors multiply the requiredsatellite power directly. A C-Band transmission requires 1.2 times thepower to overcome rain loss, while a Ku-Band transmission requires 6.3times to overcome rain loss.

An added effect comes from noise radiation due to the rain itself.Without the absorption the receiver has a temperature at C-Band that isa little better than that available at Ku-Band. C-Band typically is 50°K. while Ku-Band is more typically 80° K. However, both noisetemperatures are affected by added radiation from rain. The relationshipis given by: ##EQU16##

T_(r) is the clear-sky receiver temperature. 50° K. for C-Band or 80° K.for Ku-Band. A_(b) is the absorption factor in algebraic terms for thedifferent frequencies, 1.2 for C-Band, 6.3 for Ku-Band. The result is##EQU17##

These values can then be used in equation (14) to determine thesatellite power P_(s) required for a given choice of modulation antennasize and frequency band. For the first example we will choose SatelliteFM TV transmitting to a 6-foot receiver at C-Band.

C-Band FM TV 6-foot antenna ##EQU18## Ku-Band FM TV 6-foot antenna##EQU19##

The above shows that C-Band into a six-foot antenna requires 16 wattsper transponder, while Ku-Band into the same size receiver requires 281watts if rain margins are accounted for.

The power levels can be expressed in Effective Isotropic Radiated Power(EIRP) using the formula:

    EIRP=10 log.sub.10 (P.sub.s ×G.sub.s)                (22)

For the U.S., coverage of G_(s) =300, the two cases become:

FM TV: C-Band: 6-foot receiver: EIRP=36.8 dBW

FM TV: Ku-Band: 6-foot receiver: EIRP=49.2 dBW

The dishes used in C-Band are larger typically than 6 feet if the weakersatellites are being received, eight, ten, and even fifteen feet areused on fringe areas where signals are weaker.

The early Ku-Band satellites for the U.S. used up to 300 watt Ku-Bandtransmitters, leading to high space segment cost and often shorttransmitter lifetime. In Europe, where each country is smaller, allowinglarger Gs, tubes of 100 watts were used in Ku-Band. Some systems haveused smaller Ku-Band antennas but then suffer signal loss when rainoccurs.

Equation (22) can be used with any of the combinations of antennas andmodulation types discussed above. In Table VI, the combinations havebeen given for the stated alternatives.

                  TABLE VI    ______________________________________    EIRP Modulation, Band & Antenna    Band  Mod.    B•C/N                           Ant. ηA.sub.r                                     T.sub.r '                                          A.sub.b                                              W     EIRP    ______________________________________    C-Band          FM-TV   24 × 10.sup.7                           72"  1.6  98   1.2 16.2  36.8                  24 × 10.sup.7                           36"  0.4  98   1.2 64.8  42.8                  24 × 10.sup.7                           18"  0.1  98   1.2 259   48.9    Ku-   FM-TV   24 × 10.sup.7                           72"  1.6  324  6.3 281   49.2    Band          24 × 10.sup.7                           36"  0.4  324  6.3 1124  55.2                  24 × 10.sup.7                           18"  0.1  324  6.3 4500  61.3    C-Band          Dig. TV 2.0 × 10.sup.7                           72"  1.6  98   1.2 1.62  26.8                  2.0 × 10.sup.7                           36"  0.4  98   1.2 6.48  32.8                  2.0 × 10.sup.7                           18"  0.1  98   1.2 25.9  38.9    Ku-   Dig. TV 2.0 × 10.sup.7                           72"  1.6  324  6.3 28.1  39.2    Band          2.0 × 10.sup.7                           36"  0.4  324  6.3 112.4 45.2                  2.0 × 10.sup.7                           18"  0.1  324  6.3 450.0 51.3    ______________________________________

The receivers available to C-Band and Ku-Band for compressed Video canbe slightly better than the parameter B·C/N assumed above, resulting inthe reception with one or two dB less EIRP than listed in Table VI. Theimprovement comes from error correcting demodulation allowing lowerB·C/N and therefore lower B·C/N.

The best approach however would appear to be to use the 36" antenna areawith about 31 dB EIRP for one compressed video channel. This allows allexisting satellites to be used.

The parameter B·C/N will increase directly proportional to the number ofchannels if multiple channels are combined on the same transponder. Twochannels double the data rate and the satellite power. Three channelstriple the data rate and satellite power.

This embodiment of the present invention uses the following designgoals:

Compressed Video C-Band EIRP:

31 dBW 1 channel

34 dBW 2 channels

35.7 dBW 3 channels

The increased number of channels with satellite EIRP has the effect ofequalizing performance for a given sized antenna that has differentsatellites in the interference nulls of the antenna. An antennareceiving a 31 dBW satellite on its main beam but with an interferer at2° away in orbit with 35.7 dBW needs three times the protection than ifthe interferer were of equal, i.e., 31 dBW, EIRP. The modulation of thepresent invention automatically compensates for this situation becauseif the central satellite has only one channel its coding gain, theprotection given by the modulation, is three times better, as required.

Advantageous Embodiments

One advantageous embodiment of an antenna for receiving a signaltransmitted from a constellation of satellites, which includes a centralsatellite and a plurality of satellites spaced at regular intervals fromthe central satellite includes two heavy attenuations matched to atleast two pairs of satellites in the constellation that are immediatelyadjacent to the central satellite, wherein the two heavy attenuationsprevent signals from the at least two pairs of adjacent satellites frominterfering with a signal being transmitted from the central satellite.

Another advantageous embodiment of an antenna for receiving a signalfrom a central satellite in a constellation of satellites, whichincludes the central satellite and a plurality of satellites spaced atregular angular intervals from the central satellite relative to theantenna, comprises: a central reflector; a first side reflector; asecond side reflector; a first effective gap between the centralreflector and the first side reflector, the first effective gap having asignificantly reduced area relative to an area of the central reflectorand an area of the first side reflector; and a second effective gapbetween the central reflector and the second side reflector, the firsteffective gap having a significantly reduced area relative to the areaof the central reflector and the area of the second side reflector, inwhich the first and second effective gaps create at least two nulls inreceived energy, which two nulls inhibit signals being transmitted fromat least two pairs of satellites in the constellation that areimmediately adjacent to the central satellite.

Another advantageous embodiment of an antenna for receiving a signalfrom a central satellite in a constellation of satellites, whichincludes the central satellite and a plurality of satellites spaced atregular angular intervals from the central satellite relative to theantenna comprises: a central reflector; a first side reflector; a secondside reflector; a first effective gap between the central reflector andthe first side reflector, the first effective gap having a significantlyreduced area relative to an area of the central reflector and an area ofthe first side reflector; and a second effective gap between the centralreflector and the second side reflector, the first effective gap havinga significantly reduced area relative to the area of the centralreflector and the area of the second side reflector, in which the firstand second effective gaps create at least two regions of heavyattenuation in received energy, which two regions of heavy attenuationinhibit signals being transmitted from at least two pairs of satellitesin the constellation that are immediately adjacent to the centralsatellite.

Another advantageous embodiment of the above antenna includes first andsecond side reflectors that are physically separate from the centralreflector.

Another advantageous embodiment of an antenna for receiving a signaltransmitted from a constellation of satellites, which includes a centralsatellite and a plurality of satellites spaced at regular angularintervals from the central satellite relative to the antenna comprises:a reflecting surface having an irregularly shaped contour that providesnormal gain for a signal from the central satellite and low gain nullsfor signals from the plurality of satellites, in which the low gainnulls prevent signals being transmitted from the plurality of satellitesfrom interfering with a signal being transmitted from the centralsatellite.

An advantageous method for receiving a signal being transmitted from aconstellation of satellites, which includes a central satellite and aplurality of satellites spaced at regular angular intervals from thecentral satellite relative to a receiving antenna, comprises the stepsof: enhancing a signal being transmitted from the central satellite witha central reflector in the receiving antenna; inhibiting interferingsignals being transmitted from the plurality of satellites by disposinga gap between a central reflector and each of two side reflectors in thereceiving antenna; and selecting a width of the gap and widths of thecentral and two side reflectors so that energy from the interferingsignals that impinges on the central reflector cancels out energy fromthe interfering signals that impinges on the two side reflectors.

Another advantageous method for receiving a signal being transmittedfrom a constellation of satellites, which includes at least a centralsatellite, a first adjacent satellite spaced from the central satelliteby a first angular interval relative to a terrestrial receiving antenna,a second adjacent satellite spaced from the central satellite by asecond angular interval that is twice the first angular interval, and athird adjacent satellite spaced from the central satellite by a thirdangular interval that is three times the first angular interval,comprises the steps of: enhancing a signal being transmitted from thecentral satellite with a central reflector in the receiving antenna;canceling interfering signals from the first, second and third adjacentsatellites by: (i) placing a gap between the central reflector and eachof two side reflectors in the receiving antenna; and (ii) selecting aneast-west dimension of the main reflector relative to an east-westdimension of the side reflector such that energy of the interferingsignals impinging upon the main reflector cancels with energy of theinterfering signals impinging upon the side reflectors.

Another advantageous method for receiving a signal being transmittedsimultaneously from a constellation of satellites, which includes atleast a central satellite, a first adjacent satellite spaced from thecentral satellite by a first angular interval relative to a terrestrialreceiving antenna, a second adjacent satellite spaced from the centralsatellite by a second angular interval that is twice the first angularinterval, and a third adjacent satellite spaced from the centralsatellite by a third angular interval that is three times the firstangular interval, comprises the steps of: enhancing a signal beingtransmitted from the central satellite with a central reflector in thereceiving antenna; canceling a first interfering signal from the firstadjacent satellite by: (i) placing a gap between the central reflectorand each of two side reflectors in the receiving antenna; and (ii)selecting an east-west dimension of the main reflector relative to aneast-west dimension of the side reflector such that energy of the firstinterfering signal impinging upon the main reflector cancels energy ofthe first interfering signal impinging upon the side reflectors;canceling a second interfering signal from the second adjacent satelliteby: (i) selecting an area of the main reflector relative to an area ofthe side reflector such that energy of the second interfering signalimpinging upon the main reflector cancels with energy of the secondinterfering signal impinging upon the side reflectors without changingthe cancellation of the first interfering signal in the second step; andcanceling a third interfering signal from the third adjacent satelliteby selecting a north-south dimension of the side reflector such that theenergy of the third interfering signal impinging upon the main reflectorcancels with energy of the third interfering signal impinging upon theside reflectors without changing the cancellation of the second or firstinterfering signals in the second or third steps.

An advantageous embodiment of the previous method occurs when the thirdstep of canceling further comprises the step of: (ii) controlling a gainof the feedhorn such that the energy of the second interfering signalimpinging upon the main reflector cancels with the energy of the secondinterfering signal impinging upon the side reflectors.

An advantageous embodiment of one of the previous methods occurs whenthe second step of canceling further comprises the step of: (ii)controlling a gain of the feedhorn such that the energy of the secondinterfering signal impinging upon the main reflector cancels with theenergy of the second interfering signal impinging upon the sidereflectors.

An advantageous method for sending a quantity of data representing avideo signal to a terrestrial antenna from a ground transmitter via amain satellite within a constellation of satellites, which includes atleast two pairs of adjacent satellites that are adjacent to the mainsatellite and spaced at regular angular intervals from the mainsatellite relative to the terrestrial antenna, comprises the steps of:compressing the quantity of data to form a quantity of compressed data;modulating the quantity of compressed data into a broadband powerefficient signal that spreads the quantity of compressed data across awide bandwidth of the ground transmitter so that the broadband powerefficient signal has 3 to 8 dB of coding gain; transmitting thebroadband power efficient signal from the ground transmitter to the mainsatellite; retransmitting the broadband power efficient signal from thesatellite; receiving the broadband power efficient signal with theterrestrial antenna; providing gain in the terrestrial antenna for thebroadband power efficient signal being retransmitted from the mainsatellite; and inhibiting signals being transmitted from the at leasttwo pairs of adjacent satellites that are independent of the signalbeing transmitted from the main satellite.

An advantageous embodiment of the previous method occurs when theseventh step of inhibiting further comprises providing a gap in theterrestrial antenna between a central reflector and two side reflectors,wherein a width of the gap and widths of the two side reflectors arematched to the regular angular intervals of the at least two pairs ofadjacent satellites.

An advantageous method for sending a video signal to a terrestrialantenna from a ground transmitter via a constellation of satellites,which includes a central satellite and a plurality of satellites spacedat regular angular intervals from the central satellite relative to theterrestrial antenna, comprises the steps of: converting the video signalinto a quantity of digital data; compressing the quantity of digitaldata to form a quantity of compressed digital data; modulating thequantity of compressed digital data into a broadband power efficientsignal that spreads the quantity of compressed digital data across abandwidth of the ground transmitter so that the broadband powerefficient signal contains 3 to 8 dB of coding gain; transmitting thebroadband power efficient signal from the ground transmitter to the mainsatellite; retransmitting the broadband power efficient signal from themain satellite; receiving the broadband power efficient signal with theterrestrial antenna; enhancing the broadband power efficient signalbeing transmitted from the central satellite with a central reflector inthe terrestrial antenna; inhibiting interfering signals beingtransmitted from the plurality of satellites by disposing a gap betweena central reflector and each of two side reflectors in the receivingantenna; and selecting a width of the gap and widths of the two sidereflectors so that energy from the interfering signals that impinges onthe central reflector cancels out energy from the interfering signalsthat impinges on the two side reflectors.

An advantageous method for sending a video signal to a terrestrialantenna from a ground transmitter via a constellation of satellites,which includes at least a central satellite, a first adjacent satellitespace from the central satellite by a first angular interval relative tothe terrestrial antenna, a second adjacent satellite spaced from thecentral satellite by a second angular interval that is twice the firstangular interval, and a third adjacent satellite spaced from the centralsatellite by a third angular interval that is three times the firstangular interval, comprises the steps of: converting the video signalinto a quantity of digital data; compressing the quantity of digitaldata to form a quantity of compressed digital data; modulating thequantity of compressed digital data into a broadband power efficientsignal that spreads the quantity of compressed digital data across awide bandwidth of the ground transmitter so that the broadband powerefficient signal contains 3 to 8 dB of coding gain; transmitting thebroadband power efficient signal from the ground transmitter to theconstellation of satellites; retransmitting the broadband powerefficient signal from the constellation of satellite; receiving thebroadband signal from the constellation of satellites with theterrestrial antenna; enhancing a broadband signal being transmitted fromthe central satellite with a central reflector in the terrestrialantenna; and canceling interfering signals from the first, second andthird adjacent satellites by: (i) placing a gap between the centralreflector and each of two side reflectors in the terrestrial antenna:and selecting an east-west dimension of the main reflector relative toan east-west dimension of the side reflector such that energy of theinterfering signals impinging upon the main reflector cancels withenergy of the interfering signals impinging upon the side reflectors.

An advantageous method for sending a video signal to a terrestrialantenna from a ground transmitter via a constellation of satellites,which includes at least a central satellite, a first adjacent satellitespaced from the central satellite by a first angular interval relativeto the terrestrial antenna, a second adjacent satellite spaced from thecentral satellite by a second angular interval that is twice the firstangular interval, and a third adjacent satellite spaced from the centralsatellite by a third angular interval that is three times the firstangular interval, comprises the steps of: converting the video signalinto a quantity of digital data; compressing the quantity of digitaldata to form a quantity of compressed digital data; modulating thequantity of compressed digital data into a broadband power efficientsignal that spreads the quantity of compressed digital data across awide bandwidth the ground transmitter so that the broadband powerefficient signal contains 3 to 8 dB of coding gain; transmitting thebroadband power efficient signal from the ground transmitter to theconstellation of satellites; retransmitting the broadband powerefficient signal from each of the satellites in the constellation ofsatellites; receiving the broadband power efficient signal with theterrestrial antenna; enhancing a main broadband power efficient signalbeing transmitted from the central satellite with a central reflector inthe terrestrial antenna; canceling a first interfering signal from thefirst adjacent satellite by: placing a gap between the central reflectorand each of two side reflectors in the terrestrial antenna; selecting aneast-west dimension of the main reflector relative to an east-westdimension of the side reflector such that energy of the firstinterfering signal impinging upon the main reflector cancels energy ofthe first interfering signal impinging upon the side reflectors; andcanceling a second interfering signal from the second adjacent satelliteby: selecting an area of the main reflector relative to an area of theside reflector such that energy of the second interfering signalimpinging upon the main reflector cancels with energy of the secondinterfering signal impinging upon the side reflectors without changingthe cancellation of the first interfering signals in the eighth step;and canceling a third interfering signal from the third adjacentsatellite by selecting a north-south dimension of the side reflectorsuch that the energy of the third interfering signal impinging upon themain reflector cancels with energy of the third interfering signalimpinging upon the side reflectors without changing the cancellation ofthe second or first interfering signals in the eighth or ninth steps.

An advantageous system for transmitting a video signal from a groundtransmitter via a main satellite within a constellation of satellites toa terrestrial antenna, comprises: a ground transmitter comprising: (i)an analog to digital converter converting the video signal to a digitalsignal; (ii) a data compressor being coupled to the analog to digitalconverter and compressing the digital signal to form a compresseddigital signal; (iii) a wideband modulator being coupled to the datacompressor and modulating the compressed digital signal into a widebandanalog shaped frequency shift keyed signal that contains 3 to 8 dB ofcoding gain; (iv) a satellite transmitter being coupled to the widebandmodulator and outputting a wideband RF signal; and (v) a satelliteantenna radiating the wideband RF signal to the main satellite at apower level such that when the wideband RF signal is retransmitted bythe main satellite and reaches the earth's surface the wideband RDsignal is at a power level that is below FCC limitations on satellitetransmissions at ground level; a terrestrial antenna having a diametersuch that a beam width of the terrestrial antenna encompasses moresatellites in the constellation of satellites than the main satellite,receiving the wideband RF signal and outputting a received signal; awideband demodulator being coupled to the terrestrial antenna anddemodulating the received signal into a received compressed digitalsignal; a data decompressor being coupled to the wideband demodulatorand converting the received compressed digital signal into a receiveddigital signal; and a digital to analog converter being coupled to thedata decompressor and converting the received digital signal into areceived video signal.

An advantageous embodiment of the previous system occurs when theterrestrial antenna further comprises: a central reflector; a first sidereflector; a second side reflector; a first gap between the centralreflector and the first side reflector; and a second gap between thecentral reflector and the second side reflector, wherein the first andsecond gaps create at least two nulls in received energy, which twonulls inhibit signals being transmitted from the at least two pairs ofadjacent satellites in the constellation.

An advantageous embodiment of the previous system occurs when thecentral reflector has a first parabolic reflecting surface, and thefirst and second side reflectors have a second parabolic reflectingsurface.

An advantageous embodiment of the previous system occurs when theterrestrial antenna further comprises a fresnel step between the centralreflector and the first and second side reflector.

An advantageous embodiment of the previous system occurs when a firstparabola defining the first parabolic reflecting surface has a firstfocal length that is shorter than a second focal length of a secondparabola defining the second parabolic reflecting surface.

An advantageous embodiment of the previous system occurs when theterrestrial antenna further comprises a feed horn, wherein the first andsecond gaps lie in an area obstructed from receiving signals from thecentral satellite by the feed horn. An advantageous embodiment of theprevious system occurs when a width of the central reflector is smallerin a north-south or vertical dimension than the first and second edgereflectors.

An advantageous embodiment of the previous system occurs when the firstand second side sections are physically separate from the main section.

An advantageous receiver for receiving a video signal being broadcastvia satellite to a terrestrial antenna as a wideband power efficientsignal, comprises: a wideband demodulator being coupled to theterrestrial antenna and demodulating a received wideband power efficientsignal being output from the terrestrial antenna, the received widebandpower efficient signal containing 3 to 8 dB of coding gain, and thewideband demodulator converting the received wideband power efficientsignal into a received compressed digital signal; a data decompressorbeing coupled to the wideband demodulator and decompressing the receivedcompressed digital signal into a received digital signal; and a digitalto analog converter being coupled to the data decompressor andconverting the received digital signal into a signal resembling thevideo signal.

A ground station for receiving a broadband power efficient RF signalbeing broadcast via a constellation of satellites to a terrestrial user,comprises: a terrestrial antenna receiving the broadband RF signal beingbroadcast from the constellation of satellites, outputting a receivedwideband power efficient signal, and having a diameter such that abeamwidth of the terrestrial antenna encompasses a target satellite inthe constellation and at least the two pairs of satellites adjacent tothe target satellite in the constellation; a wideband demodulator beingcoupled to the terrestrial antenna and demodulating the receivedwideband power efficient signal being output from the terrestrialantenna into a received compressed digital signal; a data decompressorbeing coupled to the wideband demodulator and decompressing the receivedcompressed digital signal into a received digital signal; and a digitalto analog converter being coupled to the data decompressor andconverting the received digital signal into a signal resembling thevideo signal being broadcast from the satellite.

A ground station for receiving a broadband power efficient RF signalbeing broadcast via a constellation of satellites to a terrestrial user,comprises: a terrestrial antenna receiving the broadband power efficientRF signal being broadcast from the constellation of satellites,outputting a received shaped frequency shift keyed signal, and having adiameter such that a beamwidth of the terrestrial antenna encompasses atarget satellite in the constellation and at least the two pairs ofsatellites adjacent to the target satellite in the constellation; ashaped frequency shift keyed demodulator being coupled to theterrestrial antenna and demodulating a received shaped frequency shiftkeyed signal being output from the terrestrial antenna into a receivedcompressed digital signal, wherein the received shaped frequency shiftkeyed signal contains 3 to 8 dB of coding gain; a data decompressorbeing coupled to the shaped frequency shift keyed demodulator anddecompressing the received compressed digital signal into a receiveddigital signal; and a digital to analog converter being coupled to thedata decompressor and converting the received digital signal into asignal resembling the video signal being broadcast from the satellite.

A ground station for receiving a television signal being broadcast as awideband power efficient RF signal from a main satellite within aconstellation of satellites, which includes at least two pairs ofadjacent satellites adjacent to the main satellite and spaced at regularangular intervals from the main satellite relative to the groundstation, the ground station comprises: a terrestrial antenna receivingthe wideband power efficient RF signal, outputting a received signal,and having a diameter such that a beamwidth of the terrestrial antennaencompasses the main satellite and the at least the two pairs ofadjacent satellites; a wideband demodulator being coupled to theterrestrial antenna and demodulating the received signal into a receivedcompressed digital signal, wherein the received signal contains 3 to 8dB of coding gain; a data decompressor being coupled to the widebanddemodulator and converting the received compressed digital signal into areceived digital signal; and a digital to analog converter being coupledto the data decompressor and converting the received digital signal intoa received television signal.

An advantageous embodiment of the previous ground station occurs whenthe terrestrial antenna further comprises: a central reflector; a firstside reflector; a second side reflector; a first gap between the centralreflector and the first side reflector; and a second gap between thecentral reflector and the second side reflector, wherein the first andsecond gaps create at least two nulls in received energy, which twonulls inhibit signals being broadcast from the at least two pairs ofadjacent satellites.

An advantageous embodiment of the previous ground station occurs whenthe central reflector has a first parabolic reflecting surface, and thefirst and second side reflectors have a second parabolic reflectingsurface.

An advantageous embodiment of the previous ground station occurs whenthe terrestrial antenna further comprises a fresnel step between thecentral reflector and the first and second side reflector.

An advantageous embodiment of the previous ground station occurs when afirst parabola defining the first parabolic reflecting surface has afirst focal length that is shorter than a second focal length of asecond parabola defining the second parabolic reflecting surface.

An advantageous embodiment of the previous ground station occurs when afirst parabola defining the first parabolic reflecting surface has afirst focal length that is different than a second focal length of asecond parabola defining the second parabolic reflecting surface.

An advantageous embodiment of the previous ground station occurs whenthe terrestrial antenna further comprises a feed horn, wherein the firstand second gaps lie in an area obstructed from receiving signals fromthe central satellite by the feed horn.

An advantageous embodiment of the previous ground station occurs when awidth of the central reflector is smaller in a north-south or verticaldimension than the first and second edge reflectors.

An advantageous embodiment of the previous ground station occurs whenthe first and second side sections are physically separate from the mainsection.

A ground station for receiving a video signal being broadcast as awideband power efficient RF signal to a terrestrial user via a mainsatellite within a constellation of satellites, which includes at leasttwo pairs of satellites adjacent to the main satellite and spaced atregular angular intervals relative to the terrestrial user, comprises: aterrestrial antenna receiving the wideband power efficient RF signal andoutputting a received signal, and including an irregularly shapedcontour that provides normal gain for a signal from the main satelliteand low gain nulls for signals from the at least two pairs ofsatellites, wherein the low gain nulls prevent signals from the at leasttwo pairs of satellites from interfering with a signal being transmittedfrom the main satellite; a shaped frequency shift keyed demodulatorbeing coupled the terrestrial antenna and demodulating the receivedsignal into a received compressed digital signal; a data decompressorbeing coupled to the shaped frequency shift keyed demodulator andconverting the received compressed digital signal into a receiveddigital signal; and a digital to analog converter being coupled to thedata decompressor and converting the received digital signal into areceived video signal available to the user.

A satellite within a constellation of satellites for retransmitting asignal to a terrestrial antenna, which has a diameter such that abeamwidth of the terrestrial antenna encompasses the satellite as wellas other satellites within the constellation of satellites, comprises: asatellite receiver receiving a shaped frequency shift keyed signal beingtransmitted from a ground transmitter, which shaped frequency shiftkeyed signal contains 3 to 8 dB of coding gain; a satellite transmitterbeing coupled to the satellite receiver and retransmitting the shapedfrequency shift keyed signal; and a satellite antenna radiating awideband RF signal at a power level such that when the wideband RFsignal reaches the earth's surface the wideband RF signal is below FCClimitations on radiated satellite power at ground level.

A geosynchronous satellite within a constellation of geosynchronoussatellites for transmitting a C-Band signal to a terrestrial antenna,which has a diameter such that a beamwidth of the terrestrial antennaencompasses the geosynchronous satellite as well as other geosynchronoussatellites within the constellation of satellites that are alsotransmitting C-Band signals, the satellite comprises: a satellitereceiver receiving a wideband power efficient RF signal from a groundtransmitter, which wideband power efficient RF signal contains 3 to 8 dBof coding gain; a satellite transmitter being coupled to the satellitereceiver and outputting the wideband power efficient RF signal at aC-Band frequency; and a satellite antenna radiating the wideband powerefficient RF signal at a power level equal to or less than 36 db EIRP.

A geosynchronous satellite within a constellation of geosynchronoussatellites for transmitting a Ku-Band signal to a terrestrial antenna,which has a diameter such that a beamwidth of the terrestrial antennaencompasses the geosynchronous satellite as well as other geosynchronoussatellites within the constellation of satellites that are alsotransmitting Ku-Band signals, the satellite comprises: a satellitereceiver receiving a wideband power efficient RF signal from a groundtransmitter, wherein the wideband power efficient RF signal contains 3to 8 dB of coding gain; a satellite transmitter being coupled to thesatellite receiver and outputting the wideband power efficient RF signalat a Ku-Band frequency; and a satellite antenna radiating the widebandpower efficient RF signal at a power level equal to or less than 48 dbEIRP.

An apparatus for transmitting a television signal to a satellite withina constellation of satellites, for broadcast back to earth to aterrestrial antenna, the apparatus comprises: an analog to digitalconverter converting the television signal to a digital televisionsignal; a data compressor being coupled to the analog to digitalconverter and compressing the digital television signal to a compresseddigital signal; a modulator modulating the compressed digital signalinto a wideband power efficient signal that contains 3 to 8 dB of codinggain; an RF transmitter being coupled to the modulator and transmittingthe wideband power efficient signal to a satellite at an RF power levelsuch that when the wideband power efficient signal is retransmitted fromthe satellite and reaches earth the wideband power efficient signal lieswithin FCC limitations on satellite transmissions at ground level.

An advantageous embodiment of the previous apparatus occurs when the RFtransmitter transmits the wideband power efficient signal to ageosynchronous satellite at a power level such that the geosynchronoussatellite retransmits the wideband power efficient signal at a powerlevel equal to or less than 36 dB EIRP and at a C-Band frequency.

An advantageous embodiment of the previous apparatus occurs when the RFtransmitter transmits the wideband power efficient signal to ageosynchronous satellite at a power level such that the geosynchronoussatellite retransmits the wideband power efficient signal at a powerlevel equal to or less than 48 dB EIRP and at a Ku-Band frequency.

What is claimed is:
 1. An antenna for receiving a signal beingtransmitted from a central satellite within a constellation ofsatellites, in which a plurality of satellites are spaced atpredetermined intervals from the central satellite, and for coupling thesignal to a receiver, said antenna comprising:a reflector having anirregularly shaped contour, defined in part by a plurality of areas cutfrom the reflector, and having a width that defines a main beam of asize that encompasses at least two pairs of satellites in theconstellation that are immediately adjacent to the central satellite oneach side; at least one feed horn coupled to the reflector; and saidfeed horn and the irregularly shaped contour including the plurality ofareas cut from the reflector forming an antenna pattern including twopairs of nulls matched to said at least two pairs of satellites in themain beam, and the two pairs of nulls preventing signals from said atleast two pairs of adjacent satellites from interfering with a signalbeing transmitted from the central satellite.
 2. An antenna forreceiving a signal from a central satellite in a constellation ofsatellites, which includes the central satellite and a plurality ofsatellites spaced at predetermined angular intervals from the centralsatellite relative to the antenna, said antenna comprising:a) a centralreflector having a center area blocked from receiving any energy; b) afirst side reflector disposed adjacent to the central reflector on oneside; c) a second side reflector disposed adjacent to the centralreflector on a side of the central reflector opposite the first sidereflector, a distance from an outer edge of the first side reflector toan outer edge of the second side reflector being of a size so that amain beam of the antenna encompasses the central satellite and at leasttwo pairs of satellites in the constellation of satellites that areimmediately adjacent to the central satellite; d) a first gap betweenthe central reflector and the first side reflector; and e) a second gapbetween the central reflector and the second side reflector,wherein saidblockage of the center area, and said first and second gaps result in anenergy distribution across said central, first side and second sidereflectors that creates at least two pairs of nulls in received energyin said main beam, which two pairs of nulls inhibit reception of signalsbeing transmitted from said at least two pairs of satellites in theconstellation that are immediately adjacent to the central satellite. 3.The antenna according to claim 2, wherein the central reflector has aparabolic reflecting surface, the first side reflector has a parabolicreflecting surface, and the second side reflector has a parabolicreflecting surface.
 4. The antenna according to claim 3, furthercomprising a fresnel step between the central reflector and the firstand second side reflector, wherein the fresnel step lies within thefirst and second gaps.
 5. The antenna according to claim 3, wherein theparabolic reflecting surface of the central reflector is defined by afirst parabola with a focal length that is shorter than a focal lengthof a second parabola that defines the parabolic reflecting surfaces ofthe first and second side reflectors.
 6. The antenna according to claim2, further comprising a feed horn, wherein the first and second gaps liein an area obstructed from receiving signals by the feed horn.
 7. Theantenna according to claim 2, wherein the central reflector is smallerin a north-south or vertical dimension than the first and second sidereflectors.
 8. An antenna for receiving a signal being transmitted fromwithin a constellation of satellites, which includes a targetedsatellite and a plurality of satellites spaced at predeterminedintervals from the targeted satellite, said antenna comprising:anirregularly shaped reflector having a plurality of areas cut from thereflector, the shaped reflector having an east-west dimension being of asize such that it defines an antenna pattern with a main beamencompassing both the targeted satellite and at least two pairs of theplurality of satellites immediately adjacent the targeted satellite; afeed horn coupled to the irregularly shaped reflector; and a combinationof the feed horn and the reflector, when viewed in the transmit mode,causing a distribution of energy across the shaped reflector such thatenergy is distributed in approximately three sections on the shapedreflector, no energy is distributed in two areas separating the threesections from each other, and no energy is distributed in a centralsection of the shaped reflector, thus forming said main beam with twopairs of heavy attenuations matched to said at least two pairs ofsatellites in the constellation.
 9. An antenna for receiving a signalfrom one of a plurality of satellites, said plurality of satellitesbeing spaced at predetermined angular intervals from each other, saidantenna comprising:a) a central reflector having a center area blockedfrom receiving any energy; b) a first side reflector disposed adjacentto the central reflector on one side; c) a second side reflectordisposed adjacent to the central reflector on a side of the centralreflector opposite the first side reflector, a distance from an outeredge of the first reflector to an outer edge of the second reflectorbeing such that a main beam of the antenna encompasses at least threepairs of satellites in addition to the one satellite; d) a firsteffective gap between the central reflector and the first sidereflector, said first effective gap having a significantly reduced arearelative to an area of the central reflector and an area of the firstside reflector; and e) a second effective gap between the centralreflector and the second side reflector, said second effective gaphaving a significantly reduced area relative to the area of the centralreflector and the area of the second side reflector, wherein saidblockage of the center area, and said first and second effective gapsresult in an energy distribution across said central, first side andsecond side reflectors that creates at least two pairs of nulls inreceived energy in a main beam that inhibit reception of signals beingtransmitted from at least three pairs of satellites in the constellationthat are immediately adjacent to the one satellite.
 10. The antennaaccording to claim 9, wherein the central reflector has a parabolicreflecting surface, the first side reflector has a parabolic reflectingsurface, and the second side reflector has a parabolic reflectingsurface.
 11. The antenna according to claim 9, further comprising afresnel step between the central reflector and the first and second sidereflector.
 12. The antenna according to claim 11, wherein the fresnelsteps lie within the first and second effective gaps.
 13. The antennaaccording to claim 9, wherein the parabolic reflecting surface of thecentral reflector is defined by a first parabola with a focal lengththat is shorter than a focal length of a second parabola that definesthe parabolic reflecting surfaces of the first and second sidereflectors.
 14. The antenna according to claim 9, further comprising afeed horn, wherein the first and second gaps lie in an area obstructedfrom receiving signals by the feed horn.
 15. An antenna for receiving asignal from a particular satellite in a geosynchronous orbit withseveral satellites spaced at predetermined angular intervals from theparticular satellite, said antenna comprising:a) a central reflectorhaving a center area blocked from receiving any energy; b) a first sidereflector disposed adjacent to the central reflector on one side; c) asecond side reflector disposed adjacent to the central reflector on aside of the central reflector opposite the first side reflector, whereina distance from an outer edge of the first side reflector to an outeredge of the second side reflector being such that a main beam of theantenna encompasses at least two pairs of satellites adjacent to theparticular satellite; d) a first effective gap between the centralreflector and the first side reflector, said first effective gap havinga significantly reduced area relative to an area of the centralreflector and an area of the first side reflector; and e) a secondeffective gap between the central reflector and the second sidereflector, said second effective gap having a significantly reduced arearelative to the area of the central reflector and the area of the secondside reflector, wherein said blockage of the center area, and said firstand second effective gaps result in an energy distribution across saidcentral, first side and second side reflectors that creates at least tworegions of heavy attenuation in received energy in a main beam of theantenna at places where signals being transmitted from said at least twopairs of satellites of the plurality of satellites that are immediatelyadjacent to the particular satellite would impinge.
 16. The antennaaccording to claim 15, wherein the first and second side reflectors arephysically separate from the central reflector.
 17. An antenna forreceiving a signal being transmitted from a constellation of satellites,which includes a desired satellite and a plurality of satellites spacedat predetermined angular intervals from the desired satellite, saidantenna comprising:a feed horn; and a reflecting surface having anirregularly shaped contour such that energy is distributed across thereflecting surface in three primary areas, a center primary area, andtwo outer primary areas, with small amounts of energy being distributedin two sections separating the center primary area from each of the twoouter primary areas and with small amounts of energy being distributedin a central section of the center primary area, whereby thisdistribution creates an antenna pattern that provides normal gain for asignal from the desired satellite and regularly repeating low gain nullsin a main beam of the antenna that encompasses more than one satellitewithin the constellation of satellites, said nulls inhibiting signalsfrom satellites other than the desired satellite.
 18. An antennacomprising:a) three reflectors having substantially equivalent surfaceareas; b) a plurality of gaps formed between the three reflectors; andc) a feed horn distributing energy across the three reflectors in apredetermined pattern, a combination of said plurality of gaps and thepredetermined pattern forming an antenna pattern including a main lobewith at least two pairs of nulls at substantially equally spacedintervals from a center of the main lobe.
 19. A C-Band antenna forreceiving a signal from satellites at two degrees separation ingeosynchronous orbit comprising:a) three reflectors with substantiallyequal surface areas, wherein a sum of the total surface areas is lessthan that of a three foot diameter parabolic dish; and b) a feed horndistributing energy across the three reflectors in a predetermined shapethus forming an antenna pattern including a main lobe with at least twopairs of nulls at approximately two degree intervals from a center ofthe main lobe.
 20. An antenna for receiving a signal from ageosynchronous satellite system in which several satellites are spacedfrom each other in predetermined intervals, said antennacomprising:means for providing gain for a signal from one of thesatellites in the geosynchronous satellite system; and means forcreating a plurality of periodically repeating nulls in a main lobe ofthe antenna for signals from each of the satellites except for thesignal from said one satellite, wherein said plurality of nulls occur atthe predetermined intervals in a far field of the antenna.
 21. Anantenna for receiving a signal from a geosynchronous satellite system inwhich several satellites are spaced from each other in regular but knownintervals, said antenna comprising:a feed horn; and a reflector couplingenergy to the feed horn, a combination of the reflector and the feedhorn forming an antenna pattern including a main lobe having a null forat least four of the closest and adjacent satellites in the system. 22.An antenna for receiving a signal from a satellite comprising:areflector having at least three primary areas, one in the middle and twoon each side separated by effective gaps; and a feed horn, when viewedin a transmit mode, distributing energy across the reflector in apredetermined pattern that includes a ring-like pattern on the middleprimary area, wherein said predetermined pattern of said feed horn andsaid effective gaps form an antenna pattern including a main lobe havinga null for at least four satellites adjacent to the satellite from whichthe signal to be received is being transmitted.
 23. An antenna forreceiving a signal from a first satellite having at least foursatellites adjacent to the first satellite comprising:a reflector havinga predetermined shape; and a feed horn, when viewed in a transmit mode,coupling energy to the reflector so that energy is distributed across anaperture of the reflector with a ring-like shape in a central area ofthe reflector and a gap on either side of the central area of thereflector so that an antenna pattern is created including a main lobehaving gain for the signal from the first satellite and a null for eachof the at least four satellites.
 24. An antenna for receiving a signalfrom within a satellite system, which includes a satellite of interestand a plurality of interfering satellites, comprising:a reflector havinga predetermined shape; and a feed horn, when viewed in a transmit mode,distributing energy across the reflector in a predetermined pattern sothat in combination with said predetermined shape and said predeterminedpattern a distribution across an aperture of the antenna includes acenter area with a ring-like shape and a gap on either side of thecenter area so that an an antenna beam includes a main lobe having atleast two pairs of nulls corresponding to locations in a direction ofthe plurality of interfering satellites.
 25. An antenna for receiving asignal from a satellite and coupling the signal to a receivercomprising:a reflector having a reflecting surface with a total surfacearea less than that of a three foot diameter dish; means for couplingthe signal to the receiver; and means for creating an antenna patternincluding a main lobe having a null for each of four satellites adjacentto the satellite from which the signal to be received is being emitted.26. An antenna for receiving a signal from a satellite comprising:atleast three reflectors having substantially similar surface areas,wherein said three reflectors are disposed in an east-west relationshiprelative to the satellite and are disposed in a predetermined spacingfrom each other; a feed horn, when viewed in a transmit mode,distributing energy across the three reflectors in a predeterminedpattern, wherein a combination of the predetermined pattern of the feedhorn and the predetermined spacing of the at least three reflectorsforms an antenna pattern including a main lobe having a null in alocation matched to a direction corresponding to each of four satellitesadjacent to the satellite from which the signal to be received is beingemitted and a gain in a location matched to a direction corresponding tothe satellite from which the signal to be received is being emitted whenthe antenna is pointed at the satellite from which the signal to bereceived is being emitted.
 27. An antenna for receiving a signal fromone satellite within a group of satellites spaced apart at regularpredetermined intervals comprising:a reflector having an east-westdimension that defines a main beam of the antenna so that said main beamincludes the one satellite and at least three other satellites withinthe group of satellites; and aperture synthesis means for inhibiting asignal from each of the at least three other satellites within the groupof satellites while simultaneously providing gain for a signal from theone satellite.
 28. The antenna according to claim 27, wherein saidaperture synthesis means comprises a shaped reflector with gaps in thereflector at predetermined locations and a blocked area in a center ofthe reflector so that little energy or no energy is received at theblocked area or in the gaps, whereby a null is formed in a main lobe ofthe antenna pattern in a direction corresponding to each of the at leastthree satellites without substantially reducing the gain in a directioncorresponding to the one satellite.
 29. The antenna according to claim28, wherein said shaped reflector further comprises three reflectorshaving substantially equivalent surface areas.
 30. The antenna accordingto claim 29, wherein the total surface area of the three reflectors isapproximately equal to that of a three foot diameter dish when used toreceive signals in the C-Band frequency range.
 31. The antenna accordingto claim 30, wherein the width of the antenna is less than approximately60 inches and the height of the antenna is less than approximately 20inches.
 32. The antenna according to claim 30, wherein the width of theantenna is less than approximately 57 inches and the height of theantenna is less than approximately 19 inches.
 33. The antenna accordingto claim 29, wherein the three reflectors comprise parabolic reflectors.34. The antenna according to claim 33, wherein the diameter of each ofthe three parabolic reflectors is less than approximately 20 inches, andthe width of the antenna is less than approximately 60 inches.
 35. Theantenna according to claim 33, wherein the diameter of each of the threeparabolic reflectors is less than approximately 19 inches, and the widthof the antenna is less than approximately 57 inches.
 36. A method forreceiving a signal from a particular satellite within a constellation ofsatellites placed in orbit at known intervals using an antenna being ofa size such that a main beam of the antenna encompasses the particularsatellite as well as at least two satellites on either side of theparticular satellite, said method comprising the steps of:a) shaping areflector of the antenna to create gaps in a surface of the reflector;and b) distributing energy across the reflector so that in combinationwith the gaps in the surface of the reflector nulls are created in amain beam of the antenna pattern that match a relative interval spacingof the at least two satellites on either side of the particularsatellite.
 37. A method for receiving a signal being transmitted at aC-band frequency from a particular satellite within a constellation ofsatellites placed in orbit at approximately 2° angular intervals usingan antenna with a reflector whose surface area is less than that of athree foot diameter dish, such that a main beam of the antennaencompasses the particular satellite as well as at least two satelliteson either side of the particular satellite, said method comprising thesteps of:a) shaping the reflector of the antenna to create gaps in thesurface of the reflector; b) distributing energy in a three primaryareas across the reflector, one in a center and one on each side of thecenter primary area, when viewed in a transmit mode; c) distributingenergy in at least the center primary area in approximately a ringshape, when viewed in a transmit mode, so that in combination with thegaps in the surface of the reflector and the distribution of energy onthe reflector nulls exist in a main beam of the antenna pattern thatmatch the 2° spacing of the at least two satellites on either side ofthe particular satellite.